Ok I'll try my best not to spoil anything.
For the Vampire side:
-You can pretty much live in the castle
-there are specific quests and items to help improve your vampire abilities
-You can get death hounds to aid you in battle
-I also think you can get some of the vamps to aid you in battle.
-personally I think it's a cooler environment
For the Dawnguard side:
-Access to Crossbows and Bolts
-Dawnguard Heavy/Light Armor( which you can also get for vamp side if you encounter DG)
-Radiant quests which could involve friends, followers and even your spouse.
-can live in the fort
-Dawnguard members will follow you after you finish the Main Quest.
-Two dogs with armor you can use as followers.
-There's an orc there who will make you blood-kin after you've done enough quests, allowing access to the Orc Holds
-Restoration Spells that directly affect Undead/Vampires
Dawnguard exclusive weaponry.
-And of course you can be van Helsing and kill vampires
So overall DG side gives more stuff, but it's really up to you. I'd play both side and see which you like better.