Does GOG have a 20% off sale? Say want you want about STEAM, but I have no problem with it or with saving money all the time.
In this case, the Steam is the better deal. The retail game apparently is DRM free, being able to play it on steam while offline (You can actually do that with some games, if you didn't know). Support of Steam Workshop, with mods and stuff, is also a killer addition.
But some games I buy on GOG, their service for old stuff is just awesome.
