its hard to say at this point... graphics will probably be like that of ps2 vs xbox... but later on ps3 will have the advantage of having better versions of games just like the xbox did over ps2.
infact, the most visually good looking game actually being PLAYED that i saw, was Gears of War for Xbox360, while the PS3 game Resistance or something whatever its called was closely followed. but then agian i expected that from the engine is running on
the best looking game was MGS4 hands down, however running in real time it may have been, it just wasnt phisically being PLAYED in any of the vids i saw.