The difference for me is DRM free and I'm able to disconnect from the social network. I don't care about achievements. With GOG I can play on an air gaped computer and I can contribute to the discussion when I want and not be surprised when some game event automatically upload to a server somewhere.
Also, the steam forums are troll infested. I don't mind some mild trolling when and if the trolls are clever but the quality of the trolling on steam is pretty poor.
