Heroes of sword or magic?

I’ll say this – the Warrior is stronger because the attack and defense bonuses, which are given much more often and apply to the entire army, are incomparable to the meager bonus to strength or knowledge that mages have (this advantage is especially noticeable on small and medium-sized maps).

It doesn’t quite align with the fact that in Heroes 3, spells like Solmyr’s “Chain Lightning” and Aislinn’s “Meteor Shower” are prohibited in battles unless a Town Hall 4 is built, which means that in Heroes 4, playing as Strength (a race without magic) is disadvantageous and difficult on many maps. In Heroes 5, the Wizard faction is a formidable force in quick games.

if the warrior could calmly learn all the same spells + while also decently upgrading his army

And the mage could upgrade the army, but worse (without artifacts) than the warrior. And the warrior (without artifacts) would cast the learned spells worse, less damage, higher cost, shorter duration.

WHY DO I NEED 20 OR MORE KNOWLEDGE IF I CAN ONLY CAST SPELLS 3-4 TIMES IN A BATTLE DUE TO MY STRENGTH?

Well, this depends entirely on skill