Good point Pete, but I think that it usually shifts between them with each new generation of GPU. Last time ATI were just ahead, before that Nvidia where just ahead……they kind of feed off eachothers technological advances!
I suppose it also depends on the developer of the game, certain games seemed to be tailored towards Nvidia cards and other towards ATI cards. And if you have the “wrong” card you can end up with glitches in the game.
I would think though that if ATI and Nvidia developed custom GPU’s for Sony or Microsoft at around the same time they would more or less be on a par with eachother in terms of performance……