I can't seem to find benchmarks comparing the two but I came across some for the 1900XTX and the 7800GTX and the 1900XTX seems to be the winner but quite a bit. I think the 7900GTX, judging by the limited increase in performance as compared to the 7800GTX that it still would fall short a little bit in performance in comparison with 1900XTX. But honestly, by no means would that difference be noticeable.
Because I am an nVidia fan, I would choose the 7900GTX.