A couple of points:
- ATI cards use less power (generally. I'm not sure how it is with the latest offerings from either side)
- Nvidia make better drivers (for Linux too)
- Nvidia generally has better opengl performance (Like Quake-based games such as Call of Duty, Medal of Honor: Allied Assault), whereas ATI have the edge (I believe) on DirectX based graphics.
I could swing either way in the future, but for now it's Nvidia for me.
