I know I'm a little late to the party on this one, but given a choice between GOG and anything else, I always choose GOG. I like what they're about - DRM free, you can download yourself a backup copy of the game you paid for, and with Galaxy you get the benefits of a client that auto updates for you and makes it easy to manage your games.
Don't get me wrong - I like and use Steam too, but I feel like they were getting close to monopoly territory and having something like GOG Galaxy out there competing with them will just make everything bettter for us, the gamers.