its analysis of the PS3 Mass Effect 2 demo -- which was apparently based on old code -- Digital Foundry has posted a lengthy comparison of the PS3 and Xbox 360 retail editions of Mass Effect 2. There's a lot of information to digest in the article, including several videos and screenshots. The upshot: Most gamers won't notice the differences between the two games, though it seems the 360 version barely edges out the PS3 one in terms of a consistent frame-rate.
The only significant difference, which boils down to personal preference, is the lighting in the two games. DF notes that the PS3 version's lighting is generally brighter than the 360 game, though not necessarily improved from a technical standpoint. The article also examines the effectiveness of the PS3's "Genesis" comic intro, as opposed to actually playing the original Mass Effect on 360.
In the end, if you just have to own the most technically impressive version, DF suggests you choose ... the PC version, of course.