He is not the best you can have. Not even close. He only levels to level 40 and doesn't equip better robes. His aoeskills can easily friendly fire adding bounty. His stats are only mediocre.
If you want a Mage-Follower I highly suggest J'zargo, who 1.) has great stats and 2.) Is the ONLY follower beside dark brotherhood initiate who has no levelcap (can reach level 81 with you).
J'zargo is the best Mage by a looooong shot.
J'Zargo is a terrible mage. He is a mediocre follower at best; as a mage he is horrible.
His stats are pointless to deliberate. His AI is awful and he casts Sparks in combat, which is just worthless. He has high skill in Heavy Armor and 1-handed, but he usually defaults to casting, so even those are wasted. Have you actually used these guys as followers, or are you just parroting what you read somewhere?
It's hard to believe that anyone would say J'Zargo is objectively a better follower. You may prefer him, but he isn't better.
Marcurio's perk set would be solid for a PC mage; for a NPC follower that's amazing.