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Morale is more important because, while traveling on the map, you can unintentionally "spoil" the morale by entering a knight's tomb or crypt that you have already visited, which will negatively affect the next battle. Also, simply maintaining an army with different alignments will affect luck. Moreover, luck cannot be spoiled in this way; furthermore, even if you intentionally lower luck (say, with the "Misfortune" spell), "Misfortune" will still not occur (WoG is not taken into account).
In short, having positive morale eliminates the possibility of negative morale occurring, and in the case of luck, only a positive effect is possible.
And even shorter:
MORALE: (+3; +2; +1; 0; -1; -2; -3) + possible unintentional change in a negative direction as a result of various activities on the map
LUCK: (+3; +2; +1; 0; 0; 0; 0) + no negative effects, as in the case of morale.
*creatures that do not have morale are not taken into account, such as the Death Knight and the Water Elemental
**positive effects of morale and luck obtained by visiting various places on the map are not taken into account, as in both cases, the effect is equivalent