I just played around with all 3 versions of the game, and the PC version is by far the best.
keep in mind this is with a 3ghz intel C2Q/9800gtx/3gm memory
-better framerate(obviously)
-ps3/360 textures look a little sharper than the PC on medium, but not by much. for example, the taxi pricing is readble but blurry on the PC version, fairly sharp on ps360, not really noticeable unless you're really looking
-many many many more vehicles on screen
-much much much better draw distance and detail distance(makes the biggest difference)
-less texture pop-in
-some graphical glitches. (flashing trees in some areas)
-trees look like trees rather than watercolor green and yellow blobs.
i suck at off-screen pics, but here is one comparison, all 3 just came out of the ped tunnel and stopped in the street.