What is more important: luck or fighting spirit?

The situation might be exactly the opposite. You might need to eliminate a unit with a single blow, for example, to prevent a hero from casting a spell. In that case, luck is definitely better.

Well, in this example, morale is actually better! :-)

Here's a different situation: if your level 7 monster has, for example, 3 hit points left, and you need to attack the last unit so as not to lose that monster. Then luck is what matters! (Well... it doesn't always work, though).