siding with either faction leads to a very permanent outcome? Like siding with the Dawnguard leads to the elimination of all vampires, and siding with the father vampire leads to permanent night? I don't know what could happen if you side with the mother. I doubt Bethesda will do this, but it might be a cool perk. You'd need to really commit to becoming a vampire at the cost of the sun.
Suddenly the Dawnguard seem like the inferior choice to me.
Of course they are inferior.Its like joining the blades other than Paarthunax. Blades=blessings to kill dragons. Paarthunax and Greybeards=find all shouts and have a dragon on your side when you fight Alduin for first time AND stay near the time-warp.