it seems to me that the only difference is that the PS3 is brighter and a little bit more smooth...But that is just one game

Brighter does not make it better, it makes it look less realistic, the shading and the shadows are horrible on the PS3 version, however the source could have the display's contrast/brightness levels set differently for each game. Other than that they look identical, but again, why would a game company market a game solely for one system, when there is more money making it for two or all three, and considering that the graphics are only going to be as good as the weakest hardware.

The differences are extremely negligible and totally dependent on the developers effort to squeeze out the most out of the respective hardware. I would even go on to say that out of any of the previous console wars this is by far the closest (in terms of graphics, of course).

At the end of the day, it's all about the GAMES and the 360 tends to have more exclusive titles ... or at least they're released on the 360 first. But once MGS4 comes out the ps3 will finally have it's killer title. But still, the ps3 is nothing more than a blueray player at this point, they need to step it up. (I say that in regards to people who own both systems, if you own neither, ps3 is my recommended choice simply because of the blue ray player)

Not necessarily. The same program (like a game) can look drastically different when run on different hardware. The program code is not responsible for shading, texturing, lighting, anti-aliasing, etc. That's all controlled by the hardware. And if the developers know what's good for them, they'll optimize the game for each system (as the differences in hardware sometimes do require different methods of coding the game to get the best performance out of it).