I actually find this to be one of the more interesting questions for the Dragonborn to choose between. On one hand, you have the Dawnguard who are a bunch of religious militants the incredible bigots of the Vigilants see as too extreme. On the other, you have a race of eternal immortals who just happen to be headed by a psychopath.

Which is the best side to go with?

Traditionally, vampires get the short end of the stick in this but the undead in Tamriel are not necessarily evil. Likewise, as head of the coven, the Dohvakiim has the potential to be a far better Vampire Father than any other. His immortality could also serve as an excellent guide to future generations of Skyrim.

Plus, as we see with Solitude's court wizard, some vampires are very productive members of society.

It comes down to what new things both of them offer.

Dawnguard = access to the strongest crossbows in the game.

Volkihar = access to Vampire Lord earlier(I think you can still get it on the DG side but much later).

The Volkihar are obviously evil. The eat still living human beings! They have people who have the sole purpose of being cattle. Their random quests also involve you killing innocent people. How are they not evil? Serana is the only one with any redeemable traits.

Generally speaking, a race of creatures sired by an evil god for the sole purpose of mocking the sanctity of death aren't exactly going to be hugging kittens and kissing babies. Apart from the handful of undead that are either pragmatic villains, willing guardians or Lawful Neutral types, most undead pretty much are heartless monsters.

Vampires, in particular, are the kings of evil undead. At least liches are content to stay in their lairs and not bother anyone.

The Dawnguard are annoying and the Volkihar are evil. Take your pick.

The Volkihar have a very nice ring with an enchantment that fortifies your Magicka by 100. A must for a mage or for an assassin that uses illusion (the best class to become a vampire because it buffs illusion, sneak and the ring with 100 more magicka is very handy for a character that doesn't always invest points in magic).

Personally, I pick the Dawnguard because Isran at least finds Serena as irritating as I do.

Actually.. it's just Lord Harkon that's evil.

I have always sided with the Dawnguard. Volkihar are just to damn evil. They don't fit well with my good natured characters.

And I love Heavy Dawnguard Armor.

Like you need me to answer that....LOL

Look at my avatar and signature.

That ring alone makes joining the Volikhar worth it for any Mage or Nightblade.