HL2 dosn't run better on nVidia cards, thats a load of pig crap. Years ago, the XTs were bundled with 'HL2 free cards,' which have now expired. HL2 will have the ATi logo all over it. While Doom 3 will have the nVidia logo all over it.
As for the XT having better benchmarks. Can you REALLY decide a card on a few FPS? According to TomsHardware benchmarks, which were taken before the 6800 raised their clock speeds, it was either very close, tied, or beat the X800 by a smidgen.
You cannot, I repeat you cannot decide a card's worth based on benchmarks alone. I have friends that have blown out their ATi cards, have had driver problems at every LAN. And these people aren't stupid. They just get problems with their cards. Before ATi stepped up in their Catalyst drivers they needed patches for Soldner, HL, and FarCry.
Reportedly, the reason Carmack went with nVidia is because it was an ATi employee that leaked the very early Doom 3 alpha that was displayed at the 2001 (maybe 2002, can't remember) E3.
For those reasons alone, I choose nVidia. By the way, their were allegations being made against the Canadian company of also cheating on their benchmarks, using similar nVidia tactics. No one picked up on these allegations, because no one cares what ATi cheats on, only what nVidia cheats on.