nVidia has a great drawback that has little to with Spec. Its that they seem to release new cards every two weeks, making whatever you buy now, redundant.

ATI is amazing for video editing, but if you can clock it properly, its more potent than the nVidia of the same calibre (and its way cheaper than nVidia)

I think that sums it up.

As for compatibility, no game is optimised for one card. Its just marketing. ATI drivers updates are not that frquent and are a somewhat hefty downloads. but that is besides the point.

One downside is that ATI tend to heat more than nVidia cards and draw more power. But the plus side is that you can tweak significantly with its performance.