I love both Steam and GoG with all my heart, but when the chips are down, and barring any significant price differences, I'll use GoG. I like the convenience of just having the game installed without the rigmarole of the Steam library (although Steam's auto-updating does balance this out). And I appreciate what GoG is doing as a company. I pre-ordered The Witcher 3 directly from GoG, partly because it was basically pre-ordering directly from the developer, but also because of their Fair Price package. For those who don't know, when GoG is forced (through publisher shenanigans) to sell a game under regional pricing, and if this significantly affects the relative price in different regions, then GoG offers free game codes to make up the difference. Essentially out of their own pocket. Which is awesome.
So yeah, GoG (marginally) wins.
I still love you, Steam.