The question of Dawnguard is not choosing whether to be a vampire lord or not, it wondering why not to be a vampire lord. Finished the vampire questline as of this post, going to work on the Dawnguard section. P.S. To blot out the sun, you litterally have to shoot it out of the sky with cursed arrows and a magical bow. The only downsides are everything seems to be seen through the eyes of a depressed mental patient and it is only active for 24 hours in-game.