Shadow/Balance would surely get the best synergy. You can both get out of form (Moonkin and Shadowform) to heal if needed, but with the DPS you should be able to tear stuff apart.
On the other hand, Feral/Shadow might work best. Sure, you don't benefit as much from the mana battery ability of the SP and he doesn't get your aura, but it's easier to take on elites or crowds of mobs with you tanking and the SP healing you, even if not specced for it.
I'd definitely go Feral/Shadow for the most part of leveling. Maybe Balance/Shadow later, but it's less appealing to me personally.