I've played as a dawnguard with one character and as a vampire in another.
It seems that the townsfolk are SAFER if I'm a vampire.
When I'm a dawnguard, the vampires will attack me upon entering a town, and kill any guards or villagers who get in the way.
But as a vampire, I get attacked by dawnguards, but the town's guards and people will protect me (not quite sure why, either because I'm a thane, or the shopkeepers like me for doing their favors or the guards just don't like people starting fights in town, maybe all three) and the dawnguards will not harm innocent townspeople, just vamps, so I can just stand there and watch the NPCs take down the dawnguards.
Gotta love how Dawnguard shows just how dumb the AI really is.