Went from AMD to nVidia (and prior to that from nVidia to AMD) and haven't noticed any difference. Personally I prefer the nVidia drivers and games have a lot more anti-aliasing options available (FXAA, CSAA, etc) that weren't available on AMD, though there's really nothing in it. I honestly just go with the card that offers the best value-to-performance at the time.
Technically speaking the current AMD drivers offer slightly lower image quality on the 7xxx series, which was discovered by Tom's Hardware. The issue will be addressed in the 12.4 drivers. But it's not like the old day when there were really obvious hacks used by both sides. nVidia still plays a bit dirty when it comes to dealing with developers (implementations that disadvantage AMD, etc), which I'd rather they didn't do.
Technically speaking the current AMD drivers offer slightly lower image quality on the 7xxx series, which was discovered by Tom's Hardware. The issue will be addressed in the 12.4 drivers. But it's not like the old day when there were really obvious hacks used by both sides. nVidia still plays a bit dirty when it comes to dealing with developers (implementations that disadvantage AMD, etc), which I'd rather they didn't do.