The civil war has created a huge debate among the entire Elder Scrolls community that I thought I would create a new debate about the vampire war (we need to come up with a better name for that).
Anyway, back to the debate. The Dawnguard in my opinion are the more logical choice in this war for a number of different reasons. The first reason is if the vampires succeed, they will be able to walk abound with mortals without fearing the sun. This will undoubtedly cause the greatest disaster Nirn has ever seen, as the vampires will be unafraid to attack the rest of the population to feed. Imagine an army of Vampire lords attacking, no one will be able to stop them.
Second, if the vampires do this, they will either kill or turn the rest of the population. Doing this, they will destroy the food source and not be able feed themselves, this is turn will make them starve to death and cause the extinction of every race.
Thirds, the leaders. Isran may be completely ignorant to the fact that there can be good vampires(however rare they are) but he has acted in a way that, he thought, was the best thing to do to protect people from the Vampire threat. Harkon on the other hand killed 1000 innocence just to become a vampire, and probably killed a lot more feeding, and is instigating a new war to kill more people.
Hope this sparks a debate that distracts people from the over used civil war debate.