It all depends on personal choice and what you going to use your PC for...
I had used both ATI and NVIDIA in the past and my personal verdict is as follows:
ATI: very good for game performance, because it was designed and redeveloped to go hand-in-hand with AMD - pure performance monsters
NVIDIA: just as good but with a added bonus - the software - awesome
You must understands that tech wise the chipsets are very close but software development complete the technical development to make a graphics awesome and a top performer
Software wise:
For me ATI's software was a bit lacking in the sense that you can't really adjust the color options to have vivid colors whereas NVIDIA's software development was perfected and give you must more room to play with the color appearance and other options on the card, but i must admit that I have not recently worked on a PC with ATI software installed so I can really comment on ATI's recent software development.
Personally i love ATI for the performance but I rather stick to NVIDIA for what their software offered me...
take care and have a nice day...