Damn, this question has been buzzing in my head for ages. Both machines are excellent, but with the introduction of the 5k Retina display, things swayed towards the new iMac pretty swiftly.
The iMac base configuration starts with an i5 CPU + AMD Radeon 2GB R9 M290X + 8GB of RAM. That configuration appears to be perfect for a less intensive use, and it feels it has been geared towards consumers rather than professionals. To that extent though this thing is expensive. Yeah, it looks awesome, and if you have that amount of cash (£2k), go for it, but it won’t offer you much more than a normal iMac apart from that gorgeous screen. ALSO, it sounds like as soon as you hit more power intensive tasks, the thing might struggle a bit to catch up.
What should I get?
If like me, video and motion graphics are what floats your boat, I’d go straight for the more expensive i7, which supports Hyper-Threading and I’d update to the more powerful graphic card, taking you to the grand total of £2.4k. £250 for 32gb of RAM are also a must, but I wouldn’t pay the Apple tax on it. It ain’t cheap, that’s for sure. If you intend to use it as a professional machine though, it's worth it.
The 5k display is a game changer and surely the best selling point of the iMac Retina. Early benchmarks and reviews shows how the combination 4.0ghz i7 + 4GB Radeon M295X is more than enough to pull off intensive tasks while moving all these pixels on screen. With this configuration even more serious gaming becomes a possibility. I only wish it would support 64gb of RAM as After Effects really needs as much as you can throw at it; that’s one for next year...
All considered, this machine is actually decent value for the money, especially against the quad-core Mac Pro. A 5k monitor will cost you around £1.5k if not more; add the cost of the computer on top of that and you get the picture.
I can’t help but thinking that the specced up version of this iMac Retina is geared towards professionals rather than consumers. I can’t see someone buying this machine for the sake of normal usage, but I can totally see this for designers, photographers, videographers and anyone else that needs monitor estate for work and a huge amount of pixels to get by. That said though, 3D artists and motion graphics designers would do with this machine, but as far as Hyper-Threading goes there are not enough cores to significantly cut render times. So, if for 2D work you are great to go, for more intensive 3D tasks I’d probably tend to suggest buying a 6 or 8 cores Mac Pro if you’ve got the cash. If not, I’d give up the Pro appellative and buy an iMac Retina, as the value of the quad-core Mac Pro is simply not good enough compared to the refreshed Apple all-in-one.