What is more important: luck or fighting spirit?

Isn't that right? +1 Morale and +1 Luck – both have a 10% chance of appearing, and 2+ have a 20% chance, respectively.

+1 Morale/Luck gives a slightly higher than 4% chance of appearing, at least that's what's written in the game mechanics, and it aligns with practice. Negative Morale appears more often than stated, about 1.5 times more.
In general (except for Necropolis), Leadership is significantly better than Luck, both in terms of additional damage and tactical advantages. But both skills are only taken out of necessity. For each race, there are 8 other more useful skills.