Carmack talks GeForces versus Radeons
 

Let me caution this by saying that this is not necessarily a benchmarked result. We've had closer relationships with Nvidia over the years, and my systems have had Nvidia cards in them for generations. We have more personal ties with Nvidia. As I understand it, ATI/AMD cards are winning a lot of the benchmarks right now for when you straight-out make synthetic benchmarks for things like that, but our games do get more hands-on polish time on the Nvidia side of things.

http://techreport.com/discussions.x/20938