Buy it from GOG for DRM free copy.
Buy on STEAM for the ecosystem like trading cards, acheivements, badges and possible built in mod support.
Reasons to buy from GOG for me
- DRM Free copy
- CD Projekt RED getting the full amount without any third party getting a cut
Reaosns for buying the game on Steam
- Part of catalog
- Easier to install patches
- Chance that GOG might give free copy with Steam key (like they did with previous two Witcher games)
Trading cards, achievements and badges are of no importance for me. With launch of GOG Galaxy it's very likely that GOG won't give free copies with Steam keys and it might make installing patches as easy as it is with Steam.
I have bought the game from GOG. Might buy the Steam version as well when it is available for $5 or less in a sale