They are pretty much the same. The 360 has a bit higher rez textures (I'm not saying this, I'm quoting it - the 360 has more available RAM) but it's unnoticeable.

I'd go with steam. I don't need lot's of functions, just a pure, easy to use, in-game browser that will work with any game.

I actually see more details on the 360 version. I don't know why... The PS3 version looks like it has a lot of small texture bugs though. PC probably has the best graphics but I do see better shading and more details on the 360 version.