nvidia, why?
1. price
2. linux support doesnt suck(ati drivers are TERRIBLE)
3. supposedly better performance.
which is best? i would probably say ati, there almost exactly as fast(can you really, REALLY with your eyes. without some meter program, see a 5fps diffrence, or even a 10fps diffrence over 50fps? it doesnt matter..), the picture is sharper, noticeably, on my nice monitor(better ramdacs?) and the stock fans dont sound like hoovers, and keep the cards from overheating(even the 6600gt has problems with this shit -_-)
so overall i would say ati, its a better overall package