Honestly Nvidia is one that you plug it in and you are good to go, ATIs are somewhat temerimental but once you iron out the kinks one is a at best 5 fps better than the other,
Nvidia does have SLI, I know ATI was working on it but have not checked back to see if one has been released so if that is important to you to have SLI,
only other thing I will say is my buddy loved Mechwarrior 4 and had an ATI Radeon VIVO and we undated his video drivers and the game would not reconize his card as a vaild card and hence would not load, on the other side my Nvidia Ti4600 I was running then never had that issue reguardless of the ver of my driver. rolling his back fixed his problem
Maybe more than you needed but if you don't have issues in possibly a little troubleshooting after driver updates then ATI and Nvidia, when comparing on equal tech levels, SLI asside of course.
I think that price wise they are not alot of difference either so it all is just what do you like or fell you trust the name more I guess