As been already said, they generally flip flop.
ATI killed the nvidia FX series and now I believe Nvidia has come up on top being the first to build and ship out a good reliable card. Way back in march of last year that card was seen as the biggest leap in graphical processing for quite some time.
IMO <--- that stands for in my OPINION, ATI rushed a release of a card to compete with it and struggled in the beginning. Haven't followed up on it since though so I don't know where it stands besides the fact I want a 6600GT which it doesn't matter in terms of nvidia or ATI, the 6600GT can be beat by either company, so with that in mind I don't have to fight over 'MINES THE MOST POWERFUL' cause it's obvious it's not