What is more important: luck or fighting spirit?

I'll take Leadership, and here's why: Luck only increases the base damage, meaning it ignores the Attack and Ranged Attack skills if the hero has them. Morale, on the other hand, doesn't ignore these skills. Moreover, Leadership allows for more flexible use of the additional damage received when Morale triggers. I'll only take Luck if the army already has +3 Morale without the Leadership skill.

Added 1 minute ago

Luck - because I mainly play as an Undead hero.

Undead heroes simply won't have a choice between Luck and Leadership. The Leadership skill never appears for them.