Steam is far more popular and better known, it has some nice features, like server stored savegames, achievements (some people are real suckers for those), friend list and in-game chats - and many others. Also, its simply convenient to have all your digital edition games in one place if you are a long time steam user (I am).That said, I'd buy a gog version myself if I wanted a digital copy (I'm getting a physical collector's edition this time), since dev support is a top factor for a fan like me, plus I already own several gog oldies and is happy with their service and ideas - but I can see why many people prefer steam.