Enough off-topic discussions in the "Reports..." thread. It's better to write here.

Here's a simple example:
A hero with a group of creatures and an attack skill (+30%) attacks an opponent with a defense skill (-15%). The base damage is 100. As a result of the attack skill, the damage is 100 * 1.3 = 130. Now, this damage is reduced by 15% due to the defense skill. We get 130 * (1 - 0.15) = 110.5. A hero with a defense skill, when attacking a hero with a protection skill, will inflict only the base damage of 100.
Conclusion: in melee combat, the one with the attack skill deals 10.5% (110.5 - 100) more damage than their opponent with defense.
However, the attack skill only increases damage for melee units (archers have their own skill - archery), while defense affects both melee and ranged units. Therefore, if there are many archers in the army, which is usually the case, I think it's better to take the defense skill. But if I'm planning to have only melee units, then I'll probably choose attack.
There were probably archers in this experiment, because when comparing two equal melee armies, the one with the hero with the attack skill should win, logically.