Vampire Lord or Vampire Hunter?
Choosing to side with the vampires means you have early access to the Vampire Lord form. This form is similar to the Werewolf form, but with different powers and the ability to revert back to human form at will. Vampirism itself is such a big topic that we actually put together a whole page on it, one you can find out at our Skyrim vampire choice page here.
You’ll also have early access to Castle Volkihar, which is full of goodies, shops, and other accommodations from the get-go. Death Hounds can be hired as dog followers, and you’ll gain access to spells and vampire-bolstering jewelry that you wouldn’t as a member of Dawnguard.
Vampire Hunter (Dawnguard)
Choosing to side with the Dawnguard gives you full access to Fort Dawnguard (which has all the normal amenities of a home base), plus weapons, armor, and spells that are effective against vampires, and the ability to hire Armored Trolls for 500g. These helpers don’t count as primary followers, so you keep your primary follower with as well. As a Dawnguard member, you’ll also have access to a few new potential human followers and can hire new husky (dog) followers, which have the properties of a normal dog follower.
Overall, the physical rewards (weapons, etc) are better if you choose Dawnguard. However, you cannot regularly use the powerful Vampire Lord form until after you’ve completed the main questline, or else the Dawnguard members will not accept you.
Note that playing as a Dawnguard member does not mean you can never become a Vampire Lord. After the main questline, and after you’ve tired of Fort Dawnguard, just ask Serana to turn you.