I voted Steam, though GOG has my heart... heh. As someone who games on the cheap, if there's any distribution channel you have to sign up for first, it's Steam. I have never paid full price there, and have well over a hundred games. Not all blockbusters, as you can imagine, but when you can get a pack of five indie games for five bucks during Christmas sales, it really takes the singe off of any purchase you may regret. They also have some major publisher collection sales and if you don't mind waiting for a game to become a year or two old you can find some really amazing deals. They're tight for multiplayer aspects too, so there's that. What you should always keep in mind though, is since it's client based and not internet independent, there may be frustrations involved. Bugs, weak net-code, even inaccessible games (this isn't scare-mongering per se, mostly difficulties with updates and weak broadband) are a possibility. Keep online mode off (no cloud save or access to social features) and update on a monthly basis to reduce niggles. Download caps and hard-drive space (you can always redownload) are factors too of course.
But GOG are really something special, and feel like a unique beacon of light for PC gaming... just well after the games have meaningful market value, is all. If you have any sense of nostalgia or you want to diversify your tastes, there's always room for a classic gem from them. None of their classic range titles (everything they offer outside of The Witcher 2) are over ten dollars, they're DRM-free, so if a game is seven years or older you should always check to see if they have it available. Their catalog certainly isn't incredibly broad though, it can be hard to find someone who releases their games with no strings attatched as DRM-free implies.
So for the poll's sake it's Steam, but since you're on The Escapist, it should really be both.
