A comparison I typed up 7 years ago that seems to have helped a few people choose between Dawnguard vs Volkihar.
Dawnguard gets:
- Pet huskies
- Pet armoured trolls
- Master Level trainers in Archery, Heavy Armor, Restoration, and Smithing.
- A couple followers
- Better crossbows
- Easier access to Dwarven and Elemental bolts
- Easier access to the spells Stendarr's Aura and Vampire's Bane
- Chance to convince Serana to cure her vampirism.
- Attacked by Volkihar Vampires. (I think) Guards/Citizens will get involved with the fight, risking their own lives to help.
Volkihar gets:
- Pet death hounds
- Master Two-Handed trainer and Expert speech trainer
- Thralls in the castle you can feed on
- The Ring of the Erudite, Ring of The Beast, Amulet of The Gargoyle, and Amulet of Bats
- The Bloodstone Chalice, to enhance your vampiric powers
- A free player home with merchants (the castle itself)
- The chance to turn your spouse into a vampire
- Easier access to Necromantic Healing and Heal Undead
- Attacked by the Danguard in the wild. Guards/citizens won't help with the fight.
Both sides get:
- Serana as a follower
- Auriel's Bow and Bloodcursed arrows
- Access to Vampire Lord form and Bloodcursed Arrows*
- Durnehviir
- Radiant quests to kill vampires**
- As long as Serana is still a vampire
** Yes, Members of Clan Volkihar will request that you go kill vampires. Don't think the Dawnguard are the only ones upset at random vampires attacking towns!