PS3 version does look better, more AA (means less jaggies, more smooth cars, people etc), slightly quicker loading due to HDD install. PS3 version also has "far less" pop-in, which is great, everything will seem to stream perfectly. Also the 360 version suffers from the occasional frame rate drop more than the PS3 does. Online gaming is also free, and the controller is more native to GTA than the 360 one.
And with 360, well if you pay up, you get extra missions. In a couple more months that is. The DLC is coming in August so it is NOT OUT NOW.
So if you're deciding which game BY ITSELF is indeed better, it goes to the PS3 version, even if it is slightly, it was more than enough for reviewers to notice.