I took the HH or well I was so Lucky to get that as first one. Shit my pants when it cast Haunt, luckily reappeared on my Darkmoon Monkey with 9hp left, dotfree phew
Both breeds are shit for speed on valkyr, so you dont want a breed that puts points into that because those points are considered 'wasted'. However the valkyr isnt meant to be speedy, Its meant to endure and be mediocre good at damage. BB offers 5% more dmg, the HH 15% more Health. Health and damage is 2 sides of same story, deal more dmg to reduce a fight by 1 round or have more Health to last another round.
Tier 1 attacks are equal, same stuff just balanced around the hit% and dmg%, you choose your luck
Tier 2 curse of doom opener, good dmg for 1 turn, gotta wait 5 rounds thou, HH breed lasts longer, siphon life could be annoying on a full Health pet but overall netgain heal+dmg isnt that high im afraid
Tier 3 offers unholy ascension, only ghost skull from dalaran sewer vendor got this, truly powerful and Unique ability, the awesome thing is you use it on the "im dead round" as a farewell
Some people would also pre-death sac themselves and apply the curse of doom to get +25% on that. Haunt has 2 bugs according to wowhead last i checked.