In a real world gaming environment, you won't notice the difference between the nVidia and the AMD competitors. In benchmarks, you see a few FPS here a few FPS there, but when you're playing a game, you're not gonna be like, "OH NO, I JUST DROPPED 2 FPS, IF ONLY I WOULD HAVE GONE WITH THE OTHER CARD".
Like bignastyid said, people buy for brand loyalty, brand name recognition (if someone recognizes the name and has heard it before, they will probably buy it), advertisement of features (nVidia tends to advertise their features a lot more; i.e. adaptive v-sync, physX, TXAA), and trust (if a product has worked for someone before, they will probably buy it again, and ATI/AMD doesn't have the best track record).
At the end of the day whichever card you choose to go with will do the same thing, play your games.