Just like AMD and Intel, nVidia and ATI go through cycles where one leads over the other. Although ATI (er...AMD now) is really focusing more on the mainstream price/performance market than the highend. Both make great cards. As for drivers, one day nVidia is lagging, the next day is ATI lagging. So it just depends.

I've got a handful of ATI cards and nVidia cards and really haven't had to much to complain about with either.

I just built my new box with an HD 3870 and have 4 wow running on them. I'll try to get some fps numbers up in the next few days...