Well, I seem to prefer ATI more than Nvidia, mainly because ATI cards (and drivers for that matter) don't seem to have problems. I've heard about a lot of problems where the driver for Geforce cards caused problems, and had to be replaced and so on and so all the people with ATI seem to be happy.

It all depends on the experiences of the user basically, and I guess the choice for the graphics card will be different for each and every one.