I want the vampire lord powers because they seem really awesome and my character is a mage but I REALLY don't like the negative effects of it. I pretty much just want the side that will benefit my magic the most. What magic am I missing out on on each side? Thanks.

Dawnguard only:

Vampire's Bane

Stendarr's Aura

Sun Fire

Vampire only:

Heal Undead

Necormantic Healing

There are only 11 new spells total in the Dawnguard DLC so neither side will probably have a huge effect on your playstyle if you're just interested in the spells. Also, I've heard that you can join the vampires then use Pacify Illusion spells on the Dawnguard members to buy stuff from them or use the wooden plate glitch to get into Caste Volkihar if you join the DG. I've never tried either method but its worth looking into if you're worried about missing out on stuff.

If you're a pure mage then a unique piece of jewellery that you obtain as a side quest for the vampires is arguably the single best unique item in the game. Better than anything you can enchant, pretty much.

You mean the http://elderscrolls.wikia.com/wiki/Ring_of_the_Erudite? I forgot about that. :/ I prefer reducing magica costs to 0 with perks and enchantments but if that doesn't suit you or your enchanting isn't high enough, you might consider joining the vampires just to get the ring. If you don't like the Vampire Lord's drawbacks, you can cure yourself pretty easily.