Xbox 360 will have better loading times due to the use of HDD install, even if it needs multiple discs. As this game is really graphicly demanding, I would think that most of the RAM of both systems will be asigned to graphics, thus the 360 will have an advantage as its dedicated graphics RAM generally is faster as far as game developers have shown so it may have better textures. The PS3 however has more powerful graphics processor, so may use more complex effects or better looking ones. The 360 probably will use AA though as it is designed to get AA for almost no penalty, you can bet that the PS3 will not as AA will just cost too much.
Processor wise, its a hard choice as its 6 simultanious threads from the 360 VS something like 7 from the cell (one core is disabled for yeld). However, I think the PS3 may have a slight advantage as it seems they love using hair physics, thus you may find the physical effects being toned down a bit for the 360.
My conclusion is, the 360 one will probably look the best from a still image, the PS3 from actual effects and frame rate. The 360 one will probably have an annoying 1-2 disc changes beeing needed as apposed to PS3's all in one blu-ray, however it makes up for that by being able to install the discs to HDD, thus it should technically have the better of the 2 loading times as blu-ray is not as fast as the 360's HDD. Thus the whole multi disc scare is nothing to worry about as new HDDs for the 360 can easilly fork out enough space for all discs at once and with the right programming from square, you may not even need to take the disc out while you are playing. The game was made for the PS3 however, so the 360 may have compatibility problems at times (low FPS or graphic bugs).
Thus both versions will have their ups and downs. I personally may get the 360 version just because I have one, however the PS3 one may be recomendable if you have that system.