I have already completed the Dawnguard side of the DLC, and I want to try the Vampire side. I have one question though: If I want to become a Vampire Lord, should I do it with my mage character that has alot of magicka, or a warrior that has alot of health?

I find the extra magicka to be more affective, but I almost never take damage as a VL. Just remember there's a bug that if Serana is with you (which she forces a lot) then your VL's Blood magic will be much much weaker.

Like Badan said having more magicka is much more affective, so having a Mage character would be recommended.