Really? I think serana felt right on the vampire side from what I have seen on youtube. Yes I have watched both sides honestly if you are on the dawnguard side and your character is a follower of meridia she would expect you to take her head right then and there. I know many people have dawnbreaker don't deny it!
Plus any vampire with a brain on their shoulders would know that blocking out the sun permanently is a dumb idea.
Well, from playing, it felt more right on the Dawnguard side for me.
The main issue on the vampire side, you never got a choice. Okay, sun blocking would be bad for future games.
Then the issue becomes, it wasn't done well or clearly. On minute I'm doing what Harkon said, then his daughter suddenly wants to go behind his back and Im alright with it. There was no real time in which it was discussed if it was a bad or good dea 'hey you should help me stop him'. It was suddenly just snapped from working with him to working against him. It wasnt written well.
Hey if I remember right, I told that chick Ill take the Dawnbreaker but I aint her champion.