Which side provides more benefits for a mage?

Vampires, mages get more bonuses and the such, same for stealth characters.

But doesn't dawnguard get the exact same benefits when they attack Castle Volkihar?

They miss one. You can only get a magic rig that gives 100 mana and regeneration speed during the vampire quests, I love it and hate t because it means i have to join vampires with every nage character or I gimp myself out on missing that ring.

As good as that sounds, I just can't bring myself to join them :/

A vampire has natural buffs that come with vamprism.