I'm an nVidia person. Yes, ATi has been performing nicely against the GeForce cards, I just stick with nVidia. I've found their cards to handle more load on them, despite them having more of a tendency to like to overheat if your cooling system blows.
Otherwise, I find the nVidia cards to be more common amonst hard core gamers, and ATi for the people who want to have the same performance or close to the GeForce cards, but want to save some money. Also, now that the GeForce 8, 9, and 200 series cards now have CUDA abilities and now have PhysX processing abilities thanks to a new driver, that makes GeForce cards even better.