Basically it comes down to cost. If you got two cards you're comparing go with the cheapest. One plus with going with an nvidia card is you're probably going to get better tech support. However I'm hoping most people on this forum don't need tech support. So it comes down to sole cost
ATI or nVidia are just companies. The deciding factor is the quality of their products, and the winning side has tended to switch every second generation within the past few years (Ti4600 - 9800XT - 6800Ultra - X1950XTX - 8800GTX - 4870X2).