Just thinking about the DLC and well there doesn't seem to sound like there is anything worth gaining from taking part in the dawnguard quest line over the vampire one as if you choose to be a vampire you seem to get countless amounts of new powers and even the possible opportunity to block out the sun!
But... the dawnguard well from the looks of it you get a crossbow and the opportunity to kill evil creatures but no amazing new powers or weapons.
so what is there to make me want to do the dawnguard questline... anyone?
The dawnguard questline isnt empty, they just have not shown it yet.
The trailer was from the vampires point of view, wait until you see more to decide the dawnguard are worthless

They wouldnt add a whole new faction without anything interesting about them.