nVidia is for me! ATi's been falling behind lately as nVidia pushes their DX10 cards more, but ATi does have some pretty good cards. I personally find better performance with the nVidia cards with the graphics maxed out.
nVidia, I have an nVidia graphics card on my laptop and it has never given me problems, except when it gets heated after playing videogames for too long. For some reason ATI is crashing in Windows with Bootcamp in an Imac I have when I try to play Doom.
I push N cards big time. However, ATI cards are not that bad, and never have been. With AMD's purchase of ATI, the v-cards will become much more reliable and competitive. My nVidia rep personally stated that AMD's purchase of ATI has sent a big scare to the suits of N.