Without spoiling anything for me, I want to use my favorite character of all time, my pure conjuration Breton mage. He's kind of a decent person but he loves the dark arts. I saw an image of Molag Bal in the trailer, and Molag Bal is my favorite Daedric lord.

I went to the fun side first: the vampires.

My first playthrough was as a dawnguard, but it was a nord who revered meridia so it was a no brainer tbh.

As for your character i think the vampire side would be a safe choice also because of molag bal since pure vampires are made by him/her (you never know with deadric princes)

Edit: grammar

Quickly, I demand a http://i.imgur.com/PeAWP.jpg fellow Molag Bal worshipper! You've answered your own question. It is obvious that you must follow the path of the vampire. Although, my character is the Champion of Molag Bal and practices the dark arts himself, I joined the Dawnguard. As I had no interest in becoming a vampire.

Vampires obviously if you like Molag Bal.