What is more important: luck or fighting spirit?

I've seen negative luck in action.

In Heroes of Might and Magic III, there are no negative luck effects, even though there's some mention that damage is halved (or something like that). They deal full damage.

That is, if the base damage of a unit is, let's say... There's a significant difference in favor of morale when comparing...

It's a small addition, or not an addition at all. For example, at the beginning of the game, if archers get morale, their ammunition runs out (there's no supply yet). However, with luck, you get double damage (or at least some additional damage). So, with luck, some shots are stronger. It's perfect for taking out zombies. ;)