ATI are best used for things that require colour accuracy rather than 3D features, I'd stick with ATI for 2D stuff without issues. Give them something that doesn't use DX & you'll see all the holes with them, GL's a fine example.

Nvidia may not have perfect drivers, but they screw up a lot less & you're not required to update the driver because a feature in a game isn't working with the one you have.

Keep in mind that the high benchmark figures you see for modern ATI stuff isn't always telling the whole story - they peak high, but drop low. Nvida stuff has a more consistent output so there's no nasty FPS drops all of a sudden. For example:

ATI 5870 in Crysis
Min 29fps Max 71fps

Seriously, a 41% performance drop. I'd stamp on a card if it did that to me. I'll stick with the 16% drop I already have TVM.