NVidia's better for gaming at the moment because their current crop of cards simply worked better. ATi's catching back up with the 38xx series, but the 2x00 series didn't work so well. If you look at the previous generation, they were evenly matched, and going back a little farther, ATi was king of the hill (the 9700/9800 series). It goes back and forth. It also seems that way because NVidia spends a lot of money with game studios to put their logo on the games and in the splash screens.
I am the Doctor, and THIS... is my SPOON!