I say get 'em both.
Out of everything out there, Oathbound is my favorite. Why? It's a DM's dream, to be able to bring anything from anywhere and it's fine. It keeps the players on their toes. And what a background it has...
Dragonstar comes in a close second, since you can import characters into it as well, though unlike Oathbound, when they return to their homeworld (if they do), they won't be the same as when they left.
As for Kalamar, I don't really care for the setting, per se, but Kenzer, in my opinion, produces the highest quality adventures, so I use them for my own homebrew campaign.
Another great plug-in for a homebrew is Bluffside. Hopefully there'll be some more adventures printed for it as well.
Chris