I just recently purchased Dawnguard and am deciding between joining them or being a vampire. I’m a mage-type player. Is being a vamp very restrictive, and would it fit being a mage better than the Dawnguard?
I’ve never actually allowed myself to become a vampire during a Skyrim playthrough. (I have, however, become a werewolf, because lycanthropy is awesome and basically has no drawbacks). So I’ll simply refer you to UESP’s page on vampirism in Skyrim, because they’re pretty much the closest you can find to an authoritative source on these sort of things.
I’ve played through Dawnguard twice, and I haven’t been a Vampire either time. I tried to be a vampire on the second play through (why I did it twice), but I couldn’t figure it out. When I was in vampire mode, I couldn’t figure out how to switch back to regular mode. And I couldn’t interact with anyone in vampire mode.
The website Smapti recommends is pretty awesome re: all things Skyrim. I’ve played up to about level 50 four times, and I still couldn’t figure it out. Oh well, I like being a werewolf. 
Interesting! I’m getting into the mods, so I might be a werebear instead just for giggles, but that sounds fun.
Begin a pure mage can be awkward as a vampire, since you won’t recover mana outdoors when the sun is out except in Vampire Lord form. Enough +mana regeneration gear can help make up for it though.
On the other hand joining the vampires gives you access to the Ring of the Erudite which is very nice for a mage. +100 Magicka and +Magicka regen. And a pure mage has little use for the Dawnguard’s crossbows.
No upsides either, really. Werewolf form is squishy as hell, can’t heal and isn’t even that much of a power upswing except in the really early game (where things die to an insistent cough anyway).
It’s nice as a travel power though - fast, you don’t get bothered by pissant brigands and the wolves leave you the hell alone. Plus it’s a permanent shield against vampirism, even after the cure 
The OP has a false choice:
After you join the Dawnguard and slaughter most of the vampires, you can ask Serana to turn you. The choice between the factions means what items you end up with though, and the vampire faction gives rings and such.
The main werewolf disadvantage is that you can’t get rested sleep. BFD I don’t necessarily want to level faster. I agree that it’s too weak sometimes, but you can heal, either passively or by gobbling down hearts. The abilities are cool, but I think that the “summon wolf/werewolf friends” is the best one by quite a bit. Werewolf excels against fewer enemies, where you can knock them down all ragdolled, and keep them from standing again.
It tells you during the tutorial, and also I think during the loading screens (but only if you are a SuperVamp). Sneak key.
Balls. Ok, I’ll have to try again.