Personally I would prefer a GOG release. That way it's DRM free and from what I understand GOG's cut is smaller than Steam's, meaning you devs get more for your effort.
Whatever you end up doing please don't let one version of the game lag behind another. There is a problem right now of some devs not updating GOG versions in favour of Steam users. Treat us all equally and we'll buy your game where it's best for both us and you :)