I've looked at other threads and most people only talk about game mechanics perks of each faction to decide which to join, couldn't really find lore talk.
If there's no official canon choice, which is more logical story-wise? I sided with the vampires and have only read up on the DG side so it's unclear to me. From what I understand, joining either faction essentially leads to the same outcome, Serana chooses to betray Harkon because of his evil plans and helps the Dragonborn stop him.
Does it make sense for the Dawnguard to forgo their ways and allow Serana to live? Harkon's evil is definitely more important but it makes it look more logical to join the vampire's side and know what it's actually like and find out for yourself how evil Harkon is and eventually replace Harkon as the vampire lord for the better, you end up helping Serana from the beginning. It seems hypocritical to join the Dawnguard then help Serana defeat her father, and then let her live then continue to be a Dawnguard.
To be honest, I find it weird for the Dragonborn to be in any type of beast form.
What I believe is canon til the the next game says is canon is the cover art of the game in the DLC Vaniila Nord male who likes a mix of Iron and Studded armor,Then his eyes are glowing and Dawngaurd so most be a vampire and Dragonborn he basically becomes Hermaeus Mora servent and wears Miraak armor. I hope he finds a way out of servitude so maybe he could be the next empreor... But I don't think Bethasda is that way so probably unkown fate like the rest.
Dawnguard is Canon. Before you go to the Soul Carin, serana asks you if you want to become a vampire so i think the dragonborn did become a vampire in the middle of the quest until the end of the quest where you get cured from falion in the end of the quest.