Well known pixel counter on neogaf.
Its basically the same differences as GTA4. 360 has AA, 720p, better framerate, less pop in. PS3 is 640p, worse framerate and more pop in. Only differences is the PS3 now has quinicrux AA (supposedly 2x) and the 360 lacks the dithering problem of GTA4.
It shouldn't be surprising. This is the same engine tht was developed for Rockstar Table Tennis. Its had far more time for refinement on the 360. The PS3 version also supports custom soundtracks, which comes with an additional RAM penalty.
He got all that from pixel counting? Damn he's good. Resolution, anyone can do, but frame rate and pop in? I wish I was him.