Oh Divines...

Here I was fully committed to being Dawnguard as a Werewolf on my first playthrough..
But damn you to hell Serena!!! Why DO YOU HAVE TO BE SO NICE AND ATTRACTIVE!?

The moment Harkon smelled Werewolf on me and burst out of his skin offering me immorality...
GAHH, im literally paused on his ultimatum with conflict!!!!

Do you really want to have it spoiled?

The consensus is that the Dawnguard side has overall better perks. You keep Serana regardless. But pick whichever side you find comfier.