What is more important: luck or fighting spirit?

Of course, it's leadership. It allows you to deal the same double damage as luck, but with the difference that you decide which of the enemy units will get the second dose of punishment. :)

No, not the same, but significantly more... :)

This is a typical misconception caused by an incorrect hint in the game. "Luck deals double damage" - it says so, but it doesn't actually correspond to reality.

It should be noted that a double turn when morale is boosted allows you to deal a full hit with a unit, while "double damage" from luck actually only doubles a small part of the damage (base damage).

That is, if the base damage of a unit is, say, 10 HP (indicated in the unit's stats), and the actual damage is, for example, 50 HP, then when morale is boosted, the unit will deal another 50, which will total 100 HP in one round of combat. And when luck is boosted, the damage will only increase by the base value and will be only 50 + 10 = 60 HP.

That is, it would be more correct to say: "Luck doubles the base damage of a unit, but does not double the bonuses provided by artifacts, spells, the secondary skill "Attack", and the hero's specialization." And bonuses always significantly exceed the base damage. This is the discrepancy...

So, there is a significant difference in favor of morale when comparing it to luck... And this topic would not have arisen if the rules of the game were described more accurately... ))

Added 5 minutes later

I had a question when I saw that it had already been raised here - does the morale (luck) indicator actually affect anything if it reaches a value above +3 or below -3?
It seemed to me that morale is less stable than luck, that is, it can be very often at +3, and sometimes it "stalls" and appears less often. Or am I just imagining things?

When the Luck/Morale indicator goes beyond the range of [-3;3], it stops affecting the probability of occurrence. Even if you collect +20, it will still be as if it were +3. As for the frequency of occurrence, a rather complex algorithm is used, which depends on many factors... But in total, with a large number of battles, the probability will exactly correspond to the calculated one... But within one battle, it may occur three times in a row, or it may not occur at all... And this is quite explainable from the point of view of probability theory...