To be honest unless you plan on actually using the Vampire Lord form a lot, you're better off going with the Dawnguard. Most of the exclusive benefits the vampires get are only active in Vampire Lord form.
Personally I enjoyed playing with the vampires more than I did the Dawnguard though.
I really enjoyed the Dawnguard. Love my crossbow, and all those elemental bolts, plus the last mission for Dawnguard was awesome!
I am not a huge fan of VL form though. Its really cool and i like that Vampires are cool, but for my playstyle, i hate being borderline useless during the day time, and hate being bothered to feed. I am much happier as a Werewolf.
Both The Vampires and Dawnguard have some very nice looking Light Armors. Both offer different colored versions of Light Armor, but the Dawnguard offers a different style for each. All the Vampire armors look the same. Dawnguard offers Black/Normal, Green/One shoulder pauldron, and Red/Sleeveless. Vampire offers Red, Black, and Grey versions of the same outfit. If your characters appearance is really important to you, the Dawnguard offer more in the way of customization in my opinion. You can get all the armors regardless of what side you choose, but I always feel weird wearing enemy faction armor. Up to you.
Only the Dawnguard offers shields-- they're both Heavy Armor shields though, so that might turn you off.
The Dawnguard offers two unique One-Handed weapons for your build. They're both hand axes though, so you might not be interested.
The best crossbow (fully upgraded) does the most single shot damage of any ranged weapon, and the elemental bolts are the icing on the cake. Granted, it doesn't fire as fast as ordinary bows so its DPS isn't as good... but that might not matter if you're trying to kill things in one hit. Something the Crossbow excels at.
The unique rewards for the Vampire Faction are related directly to the Vampire Lord form and make it more powerful-- so if you don't plan on using the Vampire Lord form a lot, you won't get the most out of the rewards.
The unique rewards for the Dawnguard Faction are three unique pieces of equipment: a two-handed hammer, a one-handed axe, and a Heavy Armor shield. Only the one-handed axe is any good in my opinion, and even then its usefulness is debatable. You also get all the upgrades for the Crossbow though, so that's pretty sweet.
Ultimately comes down to this question: Vampire Lord boosts or Awesome Crossbow. Which do you want?