The GTO is getting harder to find lately. I doubt it'll last for more than a few weeks in stock, at this price anyway.
Performance wise, the reviews prefer the GTO (not by much, but still).
However, the 7950GT (usually) supports HDCP ... which will be handy for Vista/blu-ray content. Then again, the cooler on the GTO is quieter and more efficient ....
Read up a few reviews on both, compare benchmars, and make an educated choice