21.5-inch iMac vs. 27-inch iMac vs 5K iMac: Which all-in-one desktop Mac should you get?

Peter Cohen · Oct 24, 2014

iMac Buyers Guide: A comparison of the different versions of Apple's razor-thin all-in-one Mac desktop

Apple's iMac rewrote the company's fortunes at a time when Apple desperately needed a hit. The year was 1998. Steve Jobs had returned to take the helm of the ailing company, but Apple had spent years churning out poorly-differentiated beige boxes. Out comes a translucent blue all-in-one computer unlike anything else on the market. It sold like wildfire and set off a design renaissance in personal computing: within a couple of years, everyone was churning out colorful, cheerful PCs that mimicked Apple's hit system.

Fast forward to the modern day: the iMac has undergone some radical changes. Gone is the translucent case; gone is the bulky CRT display. In their place is an incredibly elegant flat panel all-in-one design that has gotten progressively thinner over the years as Apple has done everything it can to get the computer itself out of the way of the computing experience. Within the iMac product line, however, there are a lot of options to consider, so let's take a look at what Apple is offering.

Comparing iMac models

The iMac comes in two different sizes: 21.5 inches and 27 inches. Most of them carry similar specifications, though there's a 27-inch model that stands apart from the rest: the 5K iMac. Let's look at what the other iMacs share:

A digital headphone jack, SDXC card slot (for transferring files from digital cameras), four USB 3 ports, two Thunderbolt ports and a Gigabit Ethernet port.

The non-Retina iMacs let you use a Thunderbolt port to power an external display at up to 2560 x 1600 pixels. So you can hook up a 30-inch display to your iMac and have a smokin' multi-monitor system. The Retina iMac supports an external display via Thunderbolt 2 at 3840 x 2160 pixels, so you can hook up a 4K display and use that in addition to the built-in 5K display.

The four iMacs priced under $2,000 also supports something Apple calls Target Display Mode, which lets you use the iMac as a monitor for another computer: for example, if you wanted to use your iMac as a big display for your MacBook Air. Because the screen on the 5K iMac is so high-resolution, Target Display Mode isn't supported on that particular model. Even the faster Thunderbolt 2 on the 5K iMac isn't quite up to the task of pushing that many pixels.

Also included on all iMacs is 802.11ac Wi-Fi networking — the faster standard that Apple's migrated most of its systems to in 2013, and which provides up to three times the data transfer speed of 802.11n. Bluetooth 4.0 also comes standard. A FaceTime HD camera is built into the bezel of the iMac.

In the fall of 2013, Apple refreshed iMacs with Intel's fourth-generation Core "Haswell" processors. While Haswell's main claim to fame has been dramatically improved battery efficiency on laptops, it has some practical applications for iMacs too: improved efficiencies increase overall CPU performance. Integrated graphics performance is significantly better than before: enough so that Apple chose to use them for the first time in the low-end iMac (more on that in a bit).

Apple's 21.5-inch iMacs make up the low end of the iMac product line. The 21.5-inch iMacs measure 20.8 inches wide and 17.7 inches high. They take up less than 7 inches of desk space from front to back; considering the screen size, they're really compact machines that can fit in a wide variety of environments.

The base model is a $1,099 21.5-inch iMac with a 1920 x 1080 pixel display, 1.4 GHz dual-core i5 processor, 8 GB RAM, 500 GB hard drive, and Intel HD 5000 integrated graphics. For $200 more you get a 2.7 GHz quad-core i5 processor, 8 GB RAM, 1 TB hard disk drive, and Intel Iris Pro graphics. For another $200 ($1,499), the 21.5-inch iMac gets a faster 2.9 GHz processor and faster Nvidia GeForce GT 750M graphics, with 1 GB of dedicated video memory.

The 27-inch iMac is bigger all around than the 21.5-inch model. It's 20.3 inches wide and 25.6 inches high, and stands about 8 inches deep on a desk. Its size may make it a slightly less flexible system to put anywhere you want, but it's a great machine on a desk, in a bedroom or a family room, and the 27-inch display can be big (and beautiful) enough to double as an entertainment system when it's not in use for something else.

The 27-inch iMac starts at $1,799. That gets you 2560 x 1440 display — the same size and resolution as Apple's Thunderbolt Display, only much thinner. A 3.2 GHz quad-core i5 processor is accompanied by 8 GB RAM, and a 1 terabyte (TB) 7200 RPM hard disk: that's faster than the 5400 RPM model included on the 21.5-inch machines. Nvidia GT 755M graphics come standard on that model. And for $200 more, you can get a 27-inch iMac with a 3.4 GHz processor and faster Nvidia GeForce GTX 775M graphics, with twice as much dedicated video memory: 2 GB.

There's also a third 27-inch model that Apple introduced in October, 2014. Priced at $2,499, it's the first Mac equipped with a 5K display: It has a screen that can display a stunning 5120 x 2880 pixels — twice the vertical resolution and twice the horizontal resolution of the other 27-inch models. Under the hood is a 3.5 GHz quad-core i5 processor, AMD Radeon R9 M290X graphics with 2 GB VRAM and a 1 TB Fusion Drive. Options let you go up to 4.0 GHz, with up to 3 TB Fusion Drive storage, 32 GB RAM and AMD Radeon R9 M295X graphics with 4 GB dedicated video RAM.

With the screen off, the 5K iMac looks much like any other Mac; it's largely equipped with the same stuff, though it's upgraded its Thunderbolt ports to Thunderbolt 2, which has twice the bandwidth (up to 20 gigabits per second) of the version found on other iMacs.

To get a sense of how the different screens compare more directly, look at this image. The smallest rectangle is proportional to the pixel resolution of the 21.5-inch iMac. The larger rectangle is proportional to the resolution of the 27-inch iMac. The largest rectangle is the 27-inch 5K iMac's screen resolution. Needless to say, the iMac scales so you can actually see what you're doing, just like the Retina MacBook Pro. 27 inches is gobs of desktop space, by the way. It's easy to lose your cursor on a screen that big with that high resolution, even on the non-Retina iMac models.

iMacs can be wall-mounted using an optional VESA adapter, but you'll need to decide this before you buy it - this is a configure-to-order option only. iMacs built before 2012 could be adapted to VESA wall mounts after the purchase by partly disassembling the case and removing the stand.

All iMacs include the Apple Wireless Keyboard and Apple's Magic Mouse. All iMacs also come standard with Apple's suite of consumer software: OS Mavericks and all the software normally included with that installation, along with iLife '13 - iMovie, iPhoto and GarageBand; as well as iWork '13 - Pages, Numbers and Keynote.

Let's drill down a bit and how you can custom-configure your iMac to make sense for what you're doing, specifically.

i5 vs. i7: Not all quad-core processors are the same

All iMacs except for the low-end $1,099 model come equipped with quad-core i5 processors — processors that can run up to four sets of instructions simultaneously. That makes them better at running applications designed to support parallel processing. Computationally-intensive software like scientific apps, 3D modeling, data transformation, even video and audio encoding (like ripping videos from DVDs or music from CDs) all benefit greatly from more cores. The low-end model, with its lower clock speed and dual-core design, isn't going to be up for the same sort of heavy lifting as its quad-core counterparts.

The high end 21.5-inch and 27-inch iMac models can be custom configured with better processors for an extra $200. The 21.5-inch model gets a 3.1 GHz quad-core Intel i7 processor. The 27-inch iMac gets a 3.5 GHz i7 processor. The 5K iMac gets a 4.0 GHz i7 processor. Not only are the clock speeds on the custom configurations slightly higher than the stock processor (with correspondingly higher burst speeds), but the i7 processor also gains an Intel technology called hyperthreading, which can make parallel processing even faster.

Apps that are optimized for OpenCL, an open standard parallel-processing framework, will benefit from the i7 processors. What's more, the discrete graphics processors on those higher end systems (GPUs like the GeForce graphics on all but the lowest-end iMac) are parallel-processing monsters. Apple's own Final Cut Pro X is an example of an OpenCL app.

Bottom line: if you plan to be doing a lot of data transformation or encoding or performing other tasks, consider bumping up the processor. For everyone else, a quad-core i5 processor clocked at the stock speeds is plenty fast for whatever you might be doing.

The dual-core on the low-end system isn't limiting for casual users who just want to surf the web and use the iMac for general purpose stuff. If you plan to do professional work on your iMac using apps that support multithreading, however, you're going to run into a CPU bottleneck pretty quickly.

Upgradable versus non-upgradeable: iMacs and RAM

8 GB comes standard on all Mac models. 8 GB is twice what the MacBook Air comes with, and what Apple includes with most Retina MacBook Pros. The 21.5-inch iMacs can be pre-configured by Apple with up to 16 GB of RAM while the 27-inch models can be pre-configured to run up to 32 GB RAM. That's because the 27-inch iMac has four separate slots that can accept RAM - twice as many as the 21.5-inch iMac.

How much RAM you need is entirely dependent what you'll be doing with your iMac, but just remember that RAM is one of the principle performance bottlenecks on your Mac. More RAM means less paging to disk with swap file data, created when your Mac has to fall back on virtual memory to work.

More RAM means you can operate more apps simultaneously without taking a performance hit and work with larger files without taking a performance hit. If you're planning on working with big Photoshop files, more RAM is a bonus. If you're planning on editing video, more RAM can help. If you're planning to use your iMac as a Digital Audio Workstation (DAW) with Garageband or Logic, more RAM will help.

There's a key difference between the 21.5-inch and 27-inch iMac worth noting. Only the 27-inch model has easily user-accessible RAM. There's a removable panel on the back of the iMac, underneath where the stand meets the display, where you can put in more RAM yourself.

Installing RAM in the 27-inch iMac is trivially easy — so easy, in fact, that Apple provides instructions on its web site. Pop a button underneath the memory compartment door and it opens. It's best to put RAM in pairs, because that doubles the memory bandwidth from 64 bits to 128 bits. That provides a bit of a performance enhancement (Apple installs memory in pairs in all iMac configurations).

So order the 21.5-inch iMac with whatever RAM you're going to need. On the 27-inch iMac, buy it stock, and shop around - you'll be able to do much better than Apple's price from reputable RAM vendors that sell Mac-compatible products.

Hard drive vs. Fusion Drive vs. flash storage: Balancing price, performance and capacity

1 TB of hard disk storage comes standard on all iMac configurations except the low-end 21.5-inch model (500 GB) and the 5K iMac (1 TB Fusion Drive). Two different types of hard disks are used on the different models: 21.5-inch iMacs get a 5400 RPM disk, while the 27-inch models get a higher-performance 7200 RPM disk. A hard disk spins around a central spindle just like the tire on the car. The difference in rotational speed means the faster drive can read and write data that much faster.

Storage options on the 21.5-inch iMac include a 1 TB Fusion Drive or a 256 GB flash drive, each for $200; or a 512 GB flash drive for $500. Any way you slice it, you're going to pay more for the same or less space — albeit much faster.

Options on the 27-inch iMac are a bit different: you can get a 3 TB 7200 hard drive for $150; a 1 TB Fusion Drive for $200 or a 3 TB Fusion Drive for $350; a 256 GB flash drive for $200, 512 GB fiash drive for $500 more, or take it all the way to 1 TB of flash storage for a very steep $1,000.

The 5K iMac comes standard with a 1 TB Fusion Drive, and Apple also makes a 3 TB Fusion Drive available, along with pure Flash storage ranging from 256 GB to 1 TB depending on your budget.

You know better than anyone what your storage needs are, and if you're coming from a laptop, it's entirely likely that 1 TB is significantly more than you had before. Having said that, stuff piles up quickly. Start editing media files on your iMac and you can run out of space very quickly, so if you're buying a 27-inch, that 3 TB upgrade can make sense.

Fusion Drive is a hybrid technology that combines 128 GB of flash storage with a hard drive mechanism, combined into one logical volume. Files that are frequently read from and written to disk remain on the flash drive, which files that are only infrequently accessed are stored on the hard drive. Anything you do that involves disk access - reading and writing files, even operations that require the Mac to page out virtual memory to disk - are going to be quicker on a Fusion Drive or a pure flash drive. It's a nice way to get the performance improvement of flash storage - which is huge - without paying the stiff premium that you do by going with pure flash.

Also, don't forget that your iMac is equipped with two Thunderbolt connections, so if you outgrow what the iMac comes with, you can attach external storage without a big hassle or a big performance penalty.

The 5K iMac sports Thunderbolt 2, which is up to twice as fast. There are specialty drive makers who produce amazing performing Thunderbolt 2-based RAID systems. Assuming you're working with 4K video on your 5K iMac, you'll probably be more interested in configuring your iMac with one of those external RAID arrays than you will be in really bulking it up internally.

My recommendation is to consider your storage needs ahead of time and figure out just how much space you actually need. Pure flash is hard on the wallet. If you're looking for better-than-average performance without paying a huge penalty, go with the Fusion Drive options. Just bear in mind unlike a laptop, it's not a huge inconvenience to add external storage to a desktop-bound iMac - it's not going anywhere.

Integrated vs. discrete graphics: Not your average Mac

The lowest-end 21.5-inch iMac sports an integrated graphics processor called Intel HD 5000. It's a similar CPU and integrated graphics setup as the MacBook Air, and it's fine for general use, though it'll be slower if you want to play games or do other stuff that taxes the Mac's graphics system.

The $1299 iMac has Iris Pro graphics - it's also built onto the Intel hardware. "Integrated graphics" gives some experienced computer users the perception of sluggish hardware that's incapable of keeping up with even routine tasks, but that's changed a lot in this generation of Intel hardware - Iris Pro graphics are quite fast, which is why Apple decided that the $1299 iMac didn't need its own discrete graphics chip anymore.

Integrated graphics differ from discrete graphics in a few key ways. One of them is that integrated graphics use the Mac's own system memory - drawing from that 8 GB well of memory that comes standard on all iMacs. Discrete graphics chips use their own dedicated video RAM instead - it's usually faster than the system memory. The Iris Pro graphics on the low-end iMac actually has 128 MB of embedded DRAM (eDRAM), which make its memory operations work considerably faster than the integrated graphics on other Haswell processors.

Nvidia graphics on the rest of the iMac line get progressively faster and more capable with each different model. At the top of the "regular" iMac food chain, Apple's $1,999 27-inch iMac has a system equipped with 2 GB of VRAM - twice what the rest of the iMacs get. What's more, there's a $150 option to get Nvidia GeForce GTX 780M graphics with 4 GB dedicated video memory. If you're looking for ultimate graphics horsepower on the iMac, that's the system to get. The 5K iMac touts graphics made by Nvidia rival AMD: The stock unit comes with Radeon R9 M290X graphics with 2 GB of dedicated video RAM; an R9 M295X with 4 GB is also an option.

Bottom line: The Iris Pro on the $1,299 iMac works faster than the discrete graphics on last year's model; it's a solid upgrade that users on a tight budget shouldn't be afraid of. It can't hold a candle to the Nvidia graphics on the rest of the iMac line, however.

Who should buy the 21.5-inch iMac?

The 21.5-inch iMac may be the lowest-priced iMac, but it's got plenty of horsepower to get a lot of work done. It's a great everyday computer - it can run anything you can throw at it with aplomb, and with a bit of extra memory and a tweak to the storage system can be a top performer for really intensive apps.

The more compact size of the 21.5-inch iMac makes it better suited to environments where space might be restricted - smaller desks, counter space and small apartments. The lower cost comes with a few shortcomings - a slower hard disk, fewer customization options - but don't let a limited budget or concerns over space dissuade you from the iMac. It's a fantastic Mac.

Just bear in mind that the 21.5-inch iMac can't be opened as easily as the 27-inch model, so order it with as much RAM as you need.

Who should buy the 27-inch iMac?

Short of the Mac Pro, the 27-inch iMac is the fastest Mac there is. Even Retina MacBook Pros that cost a lot more don't work as fast. The combination of speedy processor, speedy storage and tons of desktop space make the iMac the most versatile and powerful Mac you can buy without spending $3000 or more.

Not everyone needs the horsepower or screen real estate of the 27-inch iMac. But those that do get a premium experience - a fast Mac tuned for maximum performance, all in an elegant enclosure that's part sculpture, part machine.

Who should buy the 5K iMac?

Not everyone needs a display that can show 14.7 million pixels, though there is a market for it. People who spend their days in digital content creation are certainly interested in the new iMac, because it's faster than the others, the screen is drop dead gorgeous, and the addition of Thunderbolt 2 doesn't hurt.

Having said that, unless you're working with 4K source material, you probably don't need the 5K iMac quite yet. But it's still a lovely machine to have, both as a productive and elegant workstation, and for a little bit of bragging rights.

Still undecided?

If you still can't make up your mind about which iMac to purchase, that's okay. Our iMac discussion forum is a great place to ask for help. You're welcome to post comments here, too.

21.5-inch iMac vs. 27-inch iMac vs 5K iMac: Which all-in-one desktop Mac should you get?

Peter Cohen