well, for each trigger, it gives +0.1 to ATB (Attack Bonus), so if 7 units with a morale of 4 trigger, it's already +0.4, while spellcasting gives a maximum of +0.3. Conclusion: morale is better...

Considering the high initiative in the late game due to minions and speed, let's assume an average initiative of 20 for the stacks, and with +5 morale, it's effectively 30 initiative. The hero's initiative is 10, so each stack will get to go 3 times before the hero goes, without considering morale. The leadership skill should trigger on average 10 times (in 21 turns). 1 * 0.1 = 1.0 ATB from morale versus 0.3 from spellcasting; in this case, morale is 3 times better than spellcasting. And considering that the initiative can be much higher, it's an incredibly powerful advantage. Of course, it's not easy to max out all of this, but the effect is very strong.