There are atleast 3 or so new armor the Dawnguard have that we can tell from screenshots and videos, and then there is Harkon's armor, but no one (except beta testers) knows if we can get that.
As far as I know, the only new armor you get is the specific Dawnguard armor if you join them. And it might be possible to steal it from Harkon if you're a Dawnguard after he's dead. I'm just assuming, don't take my word for it.
Dawnguard have at least 2 armors you can get, vampire players will get some clothing/armor (seen in the trailer) and Harkon's armor is probably also something we can get.