Well currently nVidia has the best card around. But it changes, soon ati comes up with a better card, then nVidia releases a new & better card...
I'd say that currently nVidia is better because they have good cards for cheap (6600GT and 6800GS) and have the best card around (7800GTX 512mb).
And I prefer nVidia. Some other ppl prefer ati, so are even so weird that claim that ati has the better card when benchmarks shows that nVidia has the best one at the moment.
And some have their reasons for liking ati and will say that ati is better at the moment, which can be true.
But if u go like "nVidia is the best because they have the best card" u might have to change it to "ati is the best because they have the best card" in a few month.
So it's really hard to say which one is better.