The Xbox 360 version currently has an edge over PS3's in our eyes, simply because its textures look deeper, although detail will be increased in both versions before the game ships in October. However, rather sparse environments and some sloppy interior visuals that smack of last-gen mean that Dark Sector probably won't be challenging Gears of War for its graphical crown of excellence.
However, there will be downloadable content for both versions and a multiplayer mode is planned (and apparently already Xbox Live enabled), so we should have a decent game on our hands once it's had a wash and brush up. In the meantime, we're off to buy a Frisbee and some electricity-conducting paint.