Honestly, I have seen games more optimized for one or the other. I think (not sure) the PS3's video card equivalent is the 7800 GTX and the Xbox is equivalent to the X1900. So basically both are almost the same. Either way they are both outdated video card and the only reason they can play games fairly well is that the games are optimized for those video card.
Also you should have posted this in the console section for more replies.