I have the Dawnguard expansion, so I was wondering whether to choose to become a Vampire Lord or a Werewolf.
Both are pretty powerful. Not as powerful as your normal form can be with Enchanting/Smithing/Alchemy and perks, but powerful enough to slaughter your enemies and be fun to play. Sprinting as a Vampire Lord/Werewolf is also the fastest way to travel in the vanilla game, I think.
Werewolves need to feed on corpses, otherwise they'll revert within 150 seconds - this is not a problem with the perk that let's you add time by feasting on animal flesh. Just devour a couple of rabbits and deer on the way to your destination, and when you arrive you have to use the Wait function to get out of the damn form because you've added so much time to your invisible timer.
Werewolves can stunlock enemies with the knockdown attack, used like a dual-wielding strike (LMB+RMB). You can even knock down Giants, it's hilarious. You can't ragdoll Mammoths but you're fast enough to stay behind them and claw their rear off.
With the perks added in Dawnguard, Werewolves can deal a ton of damage. Your strongest attack is the lunge attack, achieved by sprinting and attacking. It's hard to pull off, as the controls are unresponsive because Bethesda. But once you pull it off you can one-hit bears before they even get to their feet, and then keep on running past their corpse, leaving it in your dust. I once slaughtered an Ancient Dragon in a few seconds after he landed with a barrage of lunge attacks. Up yours Akatosh, Hircine all the way.
Vampire Lords don't have to bother with feeding on corpses, and have the ability to attack at range, but deal much less damage. Your Vampiric Drain restores Health though, and by playing the Volkihar (vampire) side in Dawnguard you get access to a magical chalice that gives you the added ability to restore Stamina and Magicka with Vampiric Drain. This makes it easier to operate in daylight if you save at least some Magicka.
Like Werewolves, they can stunlock most enemies with Vampiric Grip. I slaughtered everyone in Riften once, and used Vampiric Grip to throw the immortal NPC's over the city walls, to keep them from bothering me. Dragons and Giants will probably be a boring spamfest to kill due to your limited ability to increase damage (much like a mage), but humanoids can be thrown around like the inferior weaklings they are with Vampiric Grip.
You're a little more versatile than Werewolf form with different powers that are basically Whirlwind Sprint Lite, Become Ethereal Lite, Unrelenting Force Lite, and Aura Whisper Lite. You'd still be better off using these shouts in your normal (armor capped, OP swords dual-wielding, potion-chugging murderhobo) form, but throwing enemies down heights with Vampiric Grip and restoring your attributes by sucking the life out of your enemies make this form worth playing.
I dissed both forms loudly before actually playing them, complaining over the forced third person perspective and the inability to loot and interact (VL form). But when I did test them I was pleasantly surprised, as both forms were powerful enough to survive and fun to play. Playing Werewolf is among the most fun I've had in Skyrim. It's a complete blessing without a single trace of a curse in it, which I don't think reflects the lore at all, but it makes for some highly entertaining gameplay.
