The question of which hero is better to play arises only in the early stages of the game, as by the end it doesn't matter – the hero uses both powerful spells and combat skills equally.
However, at the very beginning, when a full-fledged army is just being formed, I believe the question of whether to play as a mage, a warrior, or a battle mage is crucial.

I can't agree. Imagine this: it's the end of the game, and a decisive battle is taking place between a mage and a warrior, or a mage-warrior and a mage (it doesn't really matter). The mage, of course, relies on magic, and the warrior relies on stats (let's also assume that the armies are roughly equal). So...
But what if the warrior remembered to grab an artifact like the Sphere of Inhibition? And what if he didn't? Then the mage will completely crush the warrior.

This is a choice that matters throughout the entire game.

And if we take into account that we are fighting different monsters, then ANYONE uses both, and the most effective approach is to combine them, because you need to be ready for anything ;)