Light.. The bonuses you get from 5 pieces of light is going to make alot of difference in the damage you do. And most light armor users utilize the skill annulment which will be your shield.

Health sounds important but its really not that useful for magicka builds.

Get at least 5 pieces light armor, this will give you 10% spell crit with the passives.
Get one heavy chest and one medium legs/shoulders/helmet/boots.

Now you will level all 3 armor classes and still have good magicka sustain.

If you have trouble surviving get the light armor skill Annulment (morph to harness magicka) and use a heal over time (HOT) skill. Always use the HOT and when you get low on health use the Annulment shield. This will prevent attacks from damaging your health and the HOT will heal you back to full. Constantly shielding is usually not necessary and only costs a lot of magicka.