
Compared to Unexpected Courage, you lose a Wild icon when you commit it to your own test, but you gain a Will icon when you commit it to another investigator's test. This is not really a great deal. One of the big strengths of Unexpected Courage is its great flexibility: it provides a pretty substantial benefit basically no matter what the game situation is. Leadership loses a lot of that. You're giving up 50% of the ability to boost yourself in exchange for the fairly modest upside of one extra Will icon. In the ideal situation, Leadership is a little bit better, but on average Unexpected Courage is going to be more useful.
So I wouldn't run this over Unexpected Courage, even in a high player count game. But of course, Unexpected Courage is a pretty good card, so "slightly worse Unexpected Courage" does not equal unplayable. You could viably run both in a 3 or 4 player game, if you could find the deck space for it. It would probably make the most sense to do so if you were playing alongside a Mystic and/or a second Guardian, as those are the classes that typically need Willpower boosts the most.