The Dawnguard won't be too receiving of a Werewolf among-st their ranks.
However, joining the Volkihar clan will force you to become a Vampire Lord...
Solution? Kill them both.
The Daedra Lords aren't exactly best brothers in a big happy family. It's not too far of a stretch for Hircine ordering you to mess with Bal's "creations". After all, Vampires are worthy prey indeed. Hunting a rival apex predator would make Hircine happy I think.
You can join the Dawnguard in the beginning and use them as a means to an end. Just don't get all beast mode in their presence and they will be none the wiser.