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. Also, Leadership allows for more flexible use of the additional damage gained from Morale. I'll only take Luck if the army already has +3 Morale without the Leadership skill.

Added 1 minute ago

Undead simply won't have a choice between Luck and Leadership. They never get the Leadership skill.

In my opinion, Luck is more useful because:
1. Morale is easier to level up.
2. As you said, Luck only increases the base damage (where did you get that from? but okay, never mind). Even if that's true, the difference will be small – just an extra 100 damage. But what is that? Essentially, nothing.
3. With Luck, you can kill in one hit. Let's imagine a situation (roughly):
5 Archangels attack 14 Mighty Gorgons. With Luck, they kill them. With Morale, 2 Gorgons remain. The Gorgons retaliate – Deadly Gaze – 1 Archangel dies. The Archangels' turn. It's not a good outcome. And this is just the simplest example.