My understanding is that FCPX works better with AMD video cards. Period. That's not to say you won't get acceleration from Nvidia cards in FCPX, but it's just better with an AMD card.
I have not heard of any significant differences in iMovie or Photos using AMD or Nvidia.
If the app you use is heavily dependent on GPU, find out which GPUs work best with the apps you use.
While AMD holds the advantage for FCPX usage, Nvidia holds the advantage with other apps, specifically apps that make use of Cuda and Adobe apps. This plus the fact that, currently, Nvidia cards generally work a little better in hackintoshes, is probably why you see lots of hackintoshes using Nvidia.
I have not read of or seen any indication that 10-series Nvidia cards will be included with the next Mac Pro.