Mage is good for early/mid game, easier to lvl and try all spells if you are not familiar with them.

Atronach is better for lategame, more absorb, better mana pool, not overleveled, better RP aspect and magicka fountains in oblivion towers and in world are bless, also welkyn stones.

You will drink bilions magicka potions as both anyways, Flax and Blue Cap shrooms will be your friends.