I believe from personal experience that Nvidia has better cards. They are usually pretty equal to be honest, but I have found the ATI cards have warped faster, I don't know maybe thats just me?
But also, Nvidia on certain games can go to a higher quality. For example; all the Source games (yes they are easy to run). My friend has a 4870 and in his Graphic menu can only go to 8 QSAA and yet, my 9600GT can go to 16 QSAA.
We also found this with Crysis and some other games.
Plus Nvidia is partnered with a lot of game companies also.