Light or Dark magic.

That's just funny.

Troops of a mage controlled by a knight, advantages:
The mage has 2% defense, attack, and leadership, while the knight has 15%, 10%, and 15%.
Very good bonuses like BB, ranged attack, resilience, and empathy.
The knight usually gets light magic, and the mage always has it in the guild.
Stats go into attack and defense, which is very important for light factions.
The artifact shop can be used to supplement missing skills (although maybe level up twilight?) and knowledge, or to enhance attack potential.
Prayer with leadership = 5 morale, i.e., practically no penalty.
Disadvantages:
Lack of mini-artifacts.
Gameplay focused on the late game.
Chaos and summon spells are not useful.
Hmm, and I think that's all.