There have been comparisons by a few sites, and the general consensus seems to be that the PS3 is the better version. This isnt taking into considering the downloadable content though, nobody knows what kind of content it will be so its not fair to judge, and nobody knows if the PS3 will get just as much content. The content could end up like Vista Ultimate Extras, super lame and never updated. The deal was that the 360 will get exclusive content, so it is coming, but it doesnt mean the PS3 cant have content too (it just cant have the 360's content).
Basically the 360 version runs at a slightly higher resolution, but the pixel shading and colors arent quite as good as on the PS3 (you can definitely see it in screenshots, the 360 version is pretty pale). The reviewers tended to prefer the PS3's graphics because the better color and HDR made a bigger difference than the resolution. The PS3 version also had less pop-in because of the installation on the HDD. If youve ever played San Andreas on the PS2 you can understand how brutal texture pop-in can be.
The 360 version also has achievements if youre into that (which means you cant really cheat... which sucks a lot of the fun out of GTA), and I imagine the multiplayer is a little better thanks to Live.
I would go PS3, mostly because of the pop-in which I hated in San Andreas. If the 360 gets some awesome downloadable content like an expansion pack with its own little story then Sony isnt just going to sit and do nothing, theyll get their own expansion as well. Plus Rockstar isnt going to let the PS3 market go untapped once the 360 contract is fulfilled, expansions are big money with popular games.