Werewolves are fun, transformation is a low duration ultimate. Still fairly underpowered. Jury is still out with new patch enhancements.

Vampires suffer no daylight penalties. Have weakness to fire and next to no health regen. Great stamina and magicka regen. Skills and ultimate are useful. Personally since the 3rd or 4th vampire nerf they are not really overpowered anymore. Some will beg to differ.

Either can be gotten by being hit by rare spawns in each factions high level zone. If you don't feel like waiting find the shrine for either (Google is your friend here) and hang around there a few minutes. Someone will either sell or give you a bite. Usually given unless you are unlucky.

Good luck!
Boss... so I take it you went werewolf. I'm told the Mammoth is easiest of the three possibilities.

I think it's doable at that level but it's a long fight with a lot of avoiding. Mammoth is easier because it's attacks are easier to avoid.