Max lvl play, BM is better single target, surv is better multi tagret/aoe.
If leveling there all fine, and I would recommend trying them all out.
bm is the strong pet dmg spec.
mm is the strong hunter hard hitting dmg spec.
surv is the "alot of small dmg that adds up spec with good multi target dmg.
Right now i would probably recommend survival mostly because of the aoe which will make killing things easier if you have access to heirloom gear. If you don't have access to heirloom gear then perhaps just go Beast mastery.