Which one would you guys use and which one would you return
Both are targeted at the midrange segment. However the 9600XT is a second generation midrange. So in this case the 6600 would be the card to keep. Especially if it is DDR2, I'm seeing some people posting great overclocks.
Iwould keep the 6600 and overclock it. (not if it's the 6600le)
it has newer technologie in it ps3+vs3 and 8pixel pipes.
They score mostly the same in games (except for doom3)