From the "communities" list on the right side of the Overview page, go to the Hardware Forum. From there, go to the Tweaker's Page, and they have PAGES with this sort of info.
With CC 2014 both the nMPro GPU's and the Imac GPU's should work with the GPU acceleration in Premiere and AME.
Even a simple $ 6300 iMac comes only with slow AMD video cards. No option for nVidia, which would be at least two times faster. Seems like a lot of money for what you get.
It is simple to get a much, much better performing Windows machine for half that amount, but everybody makes his/her own choice.
General reading and looking at others experience seems to show CUDA capable cards are better. This maybe down to a faster and more stable CUDA implementation in Adobe applications compared to the OpenCL equivalent. Make sure you test the hardware and software combination you want to use in a setting which allows you to return one or the other if compatibility is not to your satisfaction.
If you are not in the 4K business --> I would go for the 32 GB & 27" iMac w/o Retina, the NVIDIA GeForce GTX 780M mit 4 GB GDDR5 (CUDA support) will give you quite a good performance, i.e. with a Thunderbolt connected Raid6 system your 2K projects (including some color correction and other plugins) will work well and you can even use the system as 4k starter/test system.
If you in the 4K business you could use a full blown iMac Retina system with a GPU AMD Radeon R9 M295X mit 4 GB GDDR5 together with an already mentioned TB Raid6 system. As described from the forum-compatriots, this non-CUDA card doesn't perform too well on GPU accelerated functions of PPro --> but --> search this forum... you will be surprised....some workflows are nearly unaffected....
If you are in the high end 4K footage busine$$ and you apply a lot of external color correction to your 4k footage (e.g. BM DaVinci) I would go for a MPro with 2 D700 cards.