What is more important: luck or fighting spirit?

I'll say this: of course, everything depends on the circumstances, as always. When you already have one of the two, then naturally, getting the second will be just great. When you have luck and morale, the army becomes the most powerful.
If we're talking specifically about choosing between the two, I would think long and hard and consider the circumstances. If I knew for sure that the opponent couldn't negatively influence my morale, I would choose morale. If I know that they can influence it, I would choose luck. In general, I prefer morale.
You know, it's fun when Dark Knights have luck. Then it's really fun. Imagine: a Dark Knight (with its ability to deal double damage) attacks with luck. Now imagine what the damage will be? Quadruple! I remember it happening once. It was fun. Those who were attacked by the knights (I don't remember who their victims were) fell from a single such blow.
Taking leadership with the Undead is complete nonsense. It's just a waste of a skill slot. Sometimes you accidentally take a scholar who gives something random, and then you get leadership. I remember it happening, and he even gave a guy spell power when he was playing as Stronghold. It was hilarious.

Added 3 minutes ago

Thirdly, morale is definitely better than luck (for those creatures that have it). Morale allows you to deal twice as much damage, while luck only allows you to deal a little more (although, sometimes, it can be significant, but less than twice as much).

How does morale allow you to deal twice as much damage?:confused: Maybe you meant to say that about luck? It's luck that allows you to deal double damage. And morale allows you to attack again, but it doesn't guarantee that the second attack will be as strong as the first. How do you know that the second attack might be weaker than the first?
Dude, you probably confused morale with luck, then your statement will be correct.;)