It has the same size hard drive, same base memory, a slightly bigger screen and a somewhat faster processor; and costs $700 more than a base model 21.5". Why then, is the 27" iMac a much better buy than its smaller brother, even in base spec?
The answer is simply value for money. You spend more of it if you buy a 27" machine but you get something that is equipped better from the outset and is more upgradeable. For starters, the base model 21.5" iMac is currently not even worth looking at. It doesn't have user upgradeable memory, it comes with a non-fusion 5400 RPM 1TB hard drive which is terribly slow, a bog standard 1080p screen (not retina), integrated graphics, a dual core (not quad core) i5 and still costs $1,000. These are not like older 21.5" iMacs which did sport user upgradeable memory and faster 7200 RPM drives before Apple slimmed the case down. In fact, in any configuration, the current 21.5" machine remains pegged down in one way or another. Any of the fusion drives you can spec will still remain as 5400 RPM 2.5" models because the machine won't fit desktop-sized hard drives. You'll get the retina screen and an i5/i7 processor worthy of the name, as well as dedicated ATI graphics if you spend more money - but then that takes you into 27" iMac territory - and who can say no to a gorgeous 5K screen?
Apple configuration page for 21.5" iMacs. In what universe does the base configuration on the left make any kind of fiscal sense?!
To start with, here are two configurations I was considering when I bought my own Mac. I had a budget of around $1,800 which would either get me a standard 27" or a kitted out 21.5".
Choice 1:
27 inch 5K screen
Core i5 3.4 GHz quad core
8GB memory
1TB fusion drive
Radeon Pro 570 4GB discrete graphics
The price for this is currently $1,799
Choice 2:
21.5 inch 4K screen
Core i7 3.6 GHz quad core
16GB memory
1TB fusion drive
Radeon Pro 560 4GB discrete graphics
The price for this is currently $1,899
So on paper, the 21.5" looks better in certain aspects when you beef it up. It has the same GPU memory (but less screen to drive), more memory, a faster processor and the same fusion drive - or does it? For starters, as mentioned before the fusion drive may be the same size, but it's not the same size. All hard drives fitted to 27" iMacs are desktop class 3.5" drives. They're faster and generally more reliable - but they don't cost any more when putting together an order with Apple. Then you have to consider the upgradeable memory - on the 27" models there is a door on the back behind the stand. You switch the Mac off, unplug power, pop the door open and slot in new memory. On the 21.5", there's no such deal, so unless you take it to Apple and pay them handsomely, you are stuck with what you ordered. The core i7 in the 21.5" configured above will be faster for sure, but how much difference it really makes depends on what you do. If you use applications that heavily rely on raw power of each core or perhaps the hyper threading that the i7 offers, then it will be pertinent. Me? I haven't noticed any real CPU related bottleneck in any of the tasks I do.
In the end it seemed like the 27" was going to be the logical choice. Only marginally slower on paper (it benches almost the same as the 3.6GHz i7 on single core tasks but around 3/4 the speed on multi core tasks) but upgradeable where it mattered and then there was the main issue to consider - the screen. For under $2,000 I would be getting an amazing 5K screen that so happened to have a competent computer attached to it. From an aesthetic angle the large bezels and metal "chin" suit the dimensions of the 27" a lot better and the top of the screen sits ideally at eye level. The colours and crispness of the screen are just gorgeous, with colours deeper than I have ever seen on another display before. The only caveat to consider is the screen is so sharp it can make even surprisingly good photographs look flawed. Video games are good (although the platform isn't principally designed for this, it does work) but again unless run at high resolution the sharpness of the screen really shows up the artefacts. I tried the Bioshock trilogy, Left 4 Dead 2 and an up to date version of Minecraft and all run well at 2048x1152 - which is higher than HD and high generally for a gaming resolution. Performance is impressive even with 8GB of memory, and as yet has not left me wanting for more, surprisingly. Disk churn is kept low, and while boot times could be better, the 45 seconds or so it takes to start most of the time isn't unacceptable.
With such a huge array of pixels on offer, scaling the screen becomes as effortless as it was back in the CRT days; something I lamented the loss of. Scaling down by one step here I have no perceptible loss in quality and still more real estate than a screen running HD at 1920x1080. Even by default the Mac runs at half 5K resolution or "pixel doubled" at 2560x1440; if the desktop was rendered in the native 5K resolution screen elements would be so small as to be nigh on impossible to see, and CPU power would be wasted rendering invisible pixels.
There are no complaints here then. The large size of the display makes viewing screen elements very easy especially when using the scaling option to make everything appear bigger (at no major loss of real estate). I considered whether I'd be missing out by taking an option of a lower processor speed, but for the last two years a dual core i5 MacBook Air was my daily. Before that an i3 PC laptop for 5 years. I never really hit the processing power ceiling in either of those much, so it seemed illogical to suddenly want gargantuan amounts of power and one must question the sanity of buying a system that will likely fail of old age before reaching obsolescence. The real nail in the coffin is while being better on paper, the 21.5" iMac that I had specced still sported the positively slothful 5400 RPM fusion drive, and would in reality have needed an upgrade to the next available SSD for acceptable performance. That would of course reduce total available storage and hike the cost higher. In the end I'm not really sure what angle Apple is taking on the 21.5"; but anyone who does their due diligence can see that it isn't a particularly savvy buy in any configuration, and for the first time in a long time they have a base model that is actually competent - the 27" 5K iMac.