“But what is the difference between: A set with a very heavy weight is going to recruit a lot of MUs; a longer set with a moderate weight is going to fatigue a lot of MUs”
the difference is waht you just stated. heavy weights recruit, mod weights fatigue (with enough volume.)
Your next question is a complicated one. Because it is a question relative to the individual.
generally speaking however, it is best to work on cross section through moderate weights as a developing athlete. Once cross section requirements are met shift to a focus more on Max strength. Of course in either case there will be work in both zones at certain time periods.