They both have the same quests, granted, they have different "mini quests" but they are the same.
Dawnguard has everything the Vampires have to offer, the transformation, everything. They only thing the Vampires have are these quests that can augment the Vampire Lord's form.
The Volkihar can get crossbows and unique spells but only by casting a Calm or Pacify spell.
It really depends on your character. I have two, one will be going to the Dawnguard and the other will become a Vampire Lord.