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 is also needed to assemble the Armor of the Elements. So, I think luck is preferable, especially for ranged units, because elves, for example, have two shots, and if they have luck, it's very effective.
In general, I try to avoid these skills; I think magical skills would be better in their place (since I usually play as warriors), like wisdom or earth/air magic.