Dawnguard or Vampire questline.

If you are used to playing in first person, playing either as a Vampire or Werewolf is a pain in the butt, since you have to play in third person. You can not fit through most doors and narrow spaces as a vampire, which is really aggravating. Neither choice is evil, so to speak, but being a blood starved vampire will make NPCs hostile to you. Being in either vampire or werewolf form will make NPCs hostile to you too. I tried playing as both, but liked being a werewolf better. Being a vampire lord gives you A LOT of perk related skills, but they are a pain to use. Most of them can only be used singularly or equipped as magic in either hand. If you are used tobeing a mage, the vampire probably would feel more natural. You can't use the skills at will, and must pick the abilities from the favorites menu. Changing to a vampire in battle is not practical, since there is a lag when you change. Also, switching back to human is a pain, since you have to pick it from the favorites menu.

In short, if you want a lot of flashy skills and magic be a vampire, if you want to be an OP melee killing machine be a werewolf. Just be prepared to clumsily stumble through the game as either.

The Dawngaurd and Vampire questlines are basically the same. You play with the same side kick with he same events. You just have to decide wether you would like to side with the vampires, or with the humans.

You get a castle either way. The vampire castle is dark, gloomy, and gothic. The Dawguard castle is sunny. The are both huge and have all the amenities available in the game.

No prob it was my pleasure.