Its usually down to how the developers create the game. Ive seen many graphical comparisons on sites where it has seemed the 360 GFX had more detail and better shadows etc. Its mainly down to the anti aliasing on the PS3 not being up to par due to the GFX chip in it. The 360 comes with built in RAM of 512Mb as opposed to PS3's 256Mb, however the RAM speed of PS3 is faster. However the 360 can produce 500 million polygons per second as opposed to the PS3's 75 million which can sometimes be a factor. Lots of developers have trouble working with the SDK tools for PS3 and therefore there can sometimes be more issues rendering things, putting detail on objects, creating shadows and certain lighting bloom effects. You will be able to tell the developers who can use the tools correctly and to decent efficiency such as Guerrilla Games (Killzone 2) and Konami (MGS 4). Its usually only the odd detail here and there so i wouldnt worry too much.