There aren’t any differences that I’m aware of. I know the Steam version has some form of Half-Life 2 tie-in like a bonus car or something like that, but I haven’t heard of anything substantial along the lines of a mission editor. Of course, with the PC version you’re more likely to see it go on sale for cheap sooner rather than later. Once Valve starts the Steam holiday sales you’ll probably be able to get it for like $5 the way they discount stuff.