id's John Carmack has posted another lenghty .plan file update, this time discussing NV30 (GeForce FX) vs R300 (Radeon 9700) among several other technical things related to DOOM 3. Here's a snip

I am using an NV30 in my primary work system now, largely so I can test more of the rendering paths on one system, and because I feel Nvidia still has somewhat better driver quality (ATI continues to improve, though). For a typical consumer, I don't think the decision is at all clear cut at the moment. For developers doing forward looking work, there is a different tradeoff -- the NV30 runs fragment programs much slower, but it has a huge maximum instruction count. I have bumped into program limits on the R300 already. As always, better cards are coming soon. ...