I don't want to give the impression that I'm ungrateful for the help that ATI offered the implementers of drivers for its cards, but I think when we take all the facts in balance it comes out in a wash or even with nvidia slightly ahead: nvidia has been consistently producing a high-quality, near-performance-parity driver for Linux for a very, very long time now, and it's almost always compatible with new kernel and Xorg versions prior to their release. ATI has given docs for basic functionality in the free drivers, but they've utterly failed to produce good, performant drivers for Linux machines, closed or free, and in fact they are now on the verge of being two Xorg releases behind.
nvidia may not have "committed to help the open-source community" until recently, but they've done a great service for Linux by being the only vendor to provide consistent, solid, high-performance drivers for Xorg and the kernel. Intel has shaped up recently and gone full-bore OSS, but nvidia has a much longer track record of good [closed] *nix support.