I always found spellcasting-based heroes to be stronger against AIs, and might-based heroes stronger against other players. There were many spells that a human opponent might be hurt by, but an AI could be absolutely abused with (see also: Slow). Since early-game battles tended to be primarily to clear out creeps, I found myself getting a better early start with a spellcaster instead of a might hero.
Pair that with the fact that spellcasting heroes could move about the map faster at high level with stuff like Summon Boat, Town Portal, Water Walk, Fly, Dimension Door letting you keep your whole stack together and yet defend or attack on several fronts at once. So I tended to favor caster heroes - not because I'd rather have a level-15 Cleric with a 4000-man army instead of a level-15 Knight with a 4000-man army, but because I'd rather have that cleric and army instead of a level-10 Knight with a 2000-man army.
And, of course, when playing the campaigns instead of multiplayer, caster heroes all the way. A high-level caster hero really doesn't even need troops to beat massive stacks of AI enemies.