What is more important: luck or fighting spirit?

Maybe you need to eliminate a unit with a single blow, for example, so that the hero doesn't have time to cast a spell. In that case, luck is definitely better.

Why aren't you considering morale in that case? It allows you to repeat an attack without passing the turn to your opponent, which will allow you to do what you intended... And the total damage will usually be higher... So there isn't a single example where luck will help you do something that morale won't... :) The conclusion and answer to the question in this thread are obvious.