The cheapest. But if everythings equal I always go with GOG and Steam is my last resort (of places I will use) if I cant get the game anywhere else at a reasonable price (or at all) then I use their service.
Humble Bundle does have all sorts of games that shouldn't be too taxing on one's system. Arcade games like Death Spank, I would hope, aren't too taking. On the precipice of Darkness another.babinro said:Humble Bundle first then the others...
I've never purchased a truly old game on humble bundle though. I'd imagine GoG is still your source for 80's and 90's software where compatibility might be a concern.