User blog comment:JAlbor/Dawnguard: What Everyone Should Know/@comment-1727531-20120701061732

Join dawnguard get hammer that plants runes, special elemental crossbow bolts and upgraded crossbow, rune axe, rune shield

Join vampire get vampire lord(both can get eventaly but ill put here), Ring of erdite +100 mana +mana regen, Ring of the beast +100 Health + claw attack, amulet of the bat (blood absorbing bats fly around you), amulet of the gargoyal (summon an extra gargoyal for 30 secs thus having 2 for half a minute and 1 for 1 minute)

Both get Auriels Bow, Auriels shield, avrak, aetherium item, ect ect

So choose dawnguard if you want one of those items on top choose vamps if you want others, story ends the same in the end just about (although the opposing faction is gutted)