It's worth noting that a double turn when morale is triggered allows a unit to deal full damage, while "double damage" from luck actually only doubles a small portion of the damage (the base damage).
In other words, if a unit's base damage is, say, 10 HP (as shown in the unit's stats), and the actual damage is, for example, 50 HP, then when morale is triggered, the unit will deal another 50, resulting in a total of 100 HP in one round of combat. But when luck is triggered, the damage will only increase by the base amount, resulting in a total of 50 + 10 = 60 HP. This is a significant difference in favor of morale when comparing the two... :)
However, there are situations where luck can be very, very useful, perhaps even better than morale, in my opinion. At the very least, it's never a bad thing to have. So... it depends on how you look at it. ;)
P.S. Maybe I've just played Heroes of Might and Magic V too much, but luck is really a good thing. :)
🏹🎪+⛓️♿=💕