What is more important: luck or fighting spirit?

Honestly, in such cases, I take Luck (both when playing as Necropolis and as Swamp – I usually play as them).
I don't quite remember if there's an artifact called "Spirit of Despair," but if I remember correctly, it removes all bonuses to defense... (Please confirm if this is true). It's often found on maps, and it's also needed to assemble the Armor of the Elements. So, I think Luck is preferable, especially for ranged units, because elves have two shots, and if they have Luck, it's amazing.
In general, I try to avoid these skills; I think magical skills can be put in their place (because I usually play as warriors), like Wisdom or Earth/Air Magic.