They have been in the past very equal, it just depended on what you prefered more. Thought ATI has been sleeping the past months a bit, Nvidia has been advancing much faster, especialy with the SLI, since it put it under a patent so ATI was forced to research something on their own and their crossover last i heard has still problems. Nvidia with the 7800 is better than ATI but its just a question of time when ATI will wake up again and get equal. All the other cards before are basicly equal and depending on which game supports which graphic company that card has than 3 or 4 frames per second more. Nvidia seems to sponsor more games thought, at least i see their logo more often in a game than ATI.
I go for Nvidia, but thats because i know already Nvidia and so far was happy with it and so dont see a need to change. As someone said its a question of taste, for me there are so small diffrences that counting them to figure out which one is better is just ridicelous.