The spec sheet doesnt say how many pixel pipelines the XT had but a reviewer said 8
While the GT has pixel 16 pipelines.
So the 6800 xt 128mb sure is cheaper, though im sure the 6800 gt is better in terms of performance because of the pixel pipelines and 256mb.
So to answer your question: 6800 GT
6800XT was nvidia answer to the x800GT. The 6800GT is clearly better card. You might want to look at 6800GS it usually cheaper then the GT and performs almost identicle.