Morale usually outweighs attack points. Positive morale improves both attack and defense with a certain percentage (+10/20/30%) as well as the crit rate (up to 140%).
So in general, the higher atk/def values a unit has, the more it gains from morale. Units of lvl 1-2 usually gain more from +atk items than + morale, because the improvement won't be that big (10% out of 4 unit attack isn't much).
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