Even at tomshardware.com the ut2004 benches showed the two cards to be pretty equal until aa and aniso were used, then the XT pulled ahead.
Call of Duty and Aquamark showed a lot of clear 68 Ultra wins however.
tomshardware benchies
My impression is that the 68Ultra will be much more future proof, but at the moment the two cards are fairly equal. Like in the above article about headroom, it just depends upon future drivers and industry support for the 6800 Ultra to win out eventually.
But if there's more ATis sold, then thats what the industry may support, so hard to tell. And by then there'll be other cards for the industry to think about. At the moment tho, hmm depends on which games ur gonna play and whether you like aa/aniso or not..