The thing is that my normal character is easily better than any werewolf transformation/vampire transformation. As such, I wanted something to modify my base character. Vampirism was the obvious way to go, and after Dawnguard, it became an even better choice. Blood potions? The Bow of Auriel? Transformations? Less severe drawbacks? 25% stronger enchantments? On top of that, my transformed state has defense (not offense) on par with my regular character. While transformed, he has over 1000 magicka (it doesn't even display properly) and over 800 health. It was anemic at first, but after sticking with it, my character has become beastly.
I chose Vampirism for the same reasons. However, in addition to them, I am also a huge fan of the Illusion magic tree in TES lore and I have kept the same build I have been running since Morrowind with an Illusion-based Assassin (more or less anyway since skills have come and go since then). Vampirism is the only permanent spell buff in the game and it just so happens it buffs Illusion spells, so this was an obvious choice for me. Really, the passive bonuses are what sold me on Vampirism. As Necrovamp noted, the Werewolf and the Vampire Lord are a ton of fun, but they will not match the raw power of your character late game. Thus, when it comes to the two of them, the passive bonuses were the deciding factor. Unfortunately, this is not a department in which the werewolf really shines.
Beyond what Necrovamp had to say, I also enjoy the versatility that comes with Vampire Lord when I do use it. Is the target at range? Use the magic. Is the target up next to me? Melee time. You could argue that the werewolves are just going to close that distance with their absurd sprint speed, but I have run into a number of cases where that range target is on an inaccessible ledge of some kind and closing that distance is impractical or an impossibility. Thus, I find the versatility more convenient when it comes to the Vampire Lord. The other thing that I like about the Vampire Lord is the default Absorb spell. That thing is among one of the more ridiculous crowd control abilities I have come across. You don't even have to hit whatever you are aiming at dead on. Just throw it in the general vicinity that your target is standing and it'll probably connect, along with anything else in range. I love that spell. Unfortunately, thanks to the Serana bugl, I cannot say the same for her companionship.