What's the best way to buy the game? I want to support you with as big of a cut as possible!
Well, my understanding is that developers get to choose what cut they get from sales made here, while Steam and GOG supposedly take ~30% of the profit(I don't think either distributor has officially disclosed this information, though, so don't quote me on that).
I don't know what the fine print is on games like this that include Steam keys or what percentage the devs chose, but you essentially get a backup copy buying here, so if nothing else that's nice.