So, let's get this console war started!
GT has a graphics comparison between the two platforms:
http://www.gametrailers.com/video/graphics-comparison-l-a-noire/714079
Honestly, I'd say even side-by-side the two are functionally indistinguishable. The 360 it looks like has some slightly more vivid colors, and the lighting/shadows on the PS3 look a little better, but even side-by-side it's barely noticeable, and you'd certainly never catch the differences in any other situation.
This is how multiplat titles should be, Rockstar, good job.