hedwards: None shall know the coming.
Seriously though, I don't think it matters one way or the other, but if you want support for something other than Windows you'd be best buying it directly from them.
As much as I like GOG, it is a Windows only site and Wasteland 2 is going to have ports available. That may or may not be a concern for you.
It would be nice to know if GOG was going to sell this, if they would I could support both Wastland 2 and GOG that way.
For the moment I'm just interested in the Windows version, although I appreciate it that they make other versions too. Still $20 isn't much for a new game these days, especially for a game that might be better than Fallout 3.
The only thing I'm worried about is that my computer is too slow for this game, but then I will buy a better computer someday anyway.