Maybe we'll end up with two product lines: iMac and iMac Pro
The iMac Pro obviously using a retina display, and an
AMD GPU which is geared towards professional photo and video work (
not gaming like Nvidia Maxwell). On the other hand there will be the standard iMac line, which is better for light gaming and basically normal computer usage.
I have a feeling that the retina 27'' model will be quite expensive, and Apple might put on a "Pro" label on it to justify the price. Then again, maybe not...who knows...
Both companies have their workstation cards in the Quadro and FirePro neither is "better" with every application and workstation cards are not the only supported cards.
Adobe tested laptop and desktop versions of the following cards. Be sure to download the latest driver for your specific model (Laptop and desktop versions have slightly different names.)
nVidia GeForce: 400, 500, 600, 700 series
nVidia Quadro: 2000, 4000 (Windows and Mac OS), CX, 5000, 6000, K600, K2000, K4000, K5000 (Windows and Mac OS)
nVidia GRID K1, K2
AMD/ATI Radeon: 5000, 6000, 7000, R7, R9 series, 7950 Mac OS
AMD/ATI FirePro: 3800, 4800, 5800, 7800, 8800, 9800, 3900, 4900, 5900, 7900, W8100, W9100
AMD/ATI FireGL: W5000, W7000, W8000
Intel HD Graphics P3000, Intel HD Graphics P4000, Intel(R) HD Graphics P4600/P4700, Intel HD Graphics 5000
Note: AMD/ATI 1000, 2000, 3000, 4000 series, nVidia GeForce 7000, 8000, 9000, 100, 200, 300 series and Intel HD Graphics (First generation) cards/GPUs are no longer being tested and are not officially supported in Photoshop CC and CC 2014. Some GL functionality is available for these cards, but it's possible that newer features don't work.
http://helpx.adobe.com/photoshop/kb/photoshop-cs6-gpu-faq1.html
This list is for Ps which doesn't use much GPU acceleration.
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The list for Premiere is here and it largely nVidia
http://helpx.adobe.com/premiere-pro/system-requirements.html
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Here is the list for Final Cut which can obviously be updated by Apple and it's largely nVidia because Apple have been using nVidia a great deal in the last couple years.
http://support.apple.com/kb/HT4664
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One of the big things done with Maxwell was improve compute. I do think though that if the MP carries AMD in the the second generation then the iMac will like carry AMD this time which would be a nice win for the red team.