What is more important: luck or fighting spirit?

In my opinion, luck is more important because:
2. As you said, luck only increases the base damage (where did you get this from? although, never mind, it doesn't matter). Even if that's true, the difference will be small – only about 100 extra damage. But what is that? Essentially, nothing.

This is taken from the damage calculation theory. Luck adds to the final damage an amount equal to the base damage without modifiers (these include attack exceeding defense, skills like archery, attack, and also the attack specialization of Crag Hack, for example). On average, the base damage is no more than 20% of the total damage, which doesn't give a very large increase. A different matter is morale, which will give at least as much damage as was already dealt in the previous attack...

3. With luck, you can kill with one hit. Let's imagine a situation (roughly speaking):
5 Archangels attack 14 Mighty Gorgons. With luck, they kill them. With morale, 2 Gorgons remain. The Gorgons retaliate – deadly gaze – 1 Archangel dies. The Archangels' turn. It's not advantageous. And this is just the simplest example.

This is not entirely a correct example because it is the only one that applies in this case. The fact is that the possible damage of a Gorgon is many times greater than its hit points. For all other units, this example will not be decisive...