[Martoon]

As a consumer, I have maybe a slight preference for nVidia, but it's pretty much a wash. They're both good GPUs, and they tend to leapfrog each other in performance with successive versions.

As a game developer, I have a strong preference for nVidia. Or rather, against ATI. Generally, nVidia chips pretty much do what they're supposed to when you use different features through DirectX or OpenGL. ATI cards almost always have some quirk that doesn't follow what the standard spec says. So we have to hack around, figuring out a way to make the ATI card do what it should have been doing in the first place. We almost always have to have special-case code to accommodate the different ATI chipsets, while the nVidia chips just do what they should. This is because ATI does a lot of "cheating" in their drivers to squeeze a little more performance out of the standard benchmark tests, by taking some performance shortcuts instead of following the spec exactly.

As a consumer, this makes no difference to you, because the developers do whatever it takes to make the game render correctly on ATI cards. But as a developer, I always cringe when it comes time to test things on the ATI cards.