The eternal question: which heroes are better, mages or warriors?

Yes, you can fight as a mage with almost no army, but the question is: at what stage of the game? Who starts with a mage who has high magical power and knowledge and knows powerful spells of levels 4-5? Give me at least one example! Yes, there's Solmyr with Chain Lightning. But for a mage to become a "really tough guy," he needs to spend at least a couple of weeks running around with a pitiful army, fighting local groups of neutral creatures. But will your opponent patiently wait for your mage to gain some experience? I doubt it! This might be possible on some custom single-player map if the author's intention is to focus on combat magic. In other cases, mages can "rule" closer to the end of the game, and even then, only if they manage to learn something "really powerful"!
I prefer to do this: I choose a warrior as the main hero, but I always try to get Wisdom, Earth Magic, and Water Magic, and depending on the situation, also Fire and Air Magic. Again, don't expect to deal a lot of damage with offensive spells: a strong melee unit will almost always deal more damage. It's better to increase Knowledge than Magical Power: you can never have too much mana!