What are the unique features of Castle Volkihar and Fort Dawnguard?
You can purchase Armored Trolls relatively early if you side with the Dawnguard's quest line. Also, you can recruit a new added in dog breed similar to a husky.
Not sure about Fort Dawnguard, but Volkihar Castle has a forge, workbench, grind stone, tanning rack, alchemy lab, and enchanting table. It can also can be upgraded for quick access to the courtyard which has many alchemy ingredients if alchemy is your thing. Oh yeah, I almost forgot about the vampire cattle if you need a quick way to feed.