I usually don't want the leadership promotion on the super-medic, as that unit will most likely not fight. It's probably going to be an old warrior/axeman/chariot, and although it can be upgraded for free would it be worth it? Having one less rifle/infantry for the bonus of healing everything in two-three turns, I think there's no contest there. If you do get an extra promotion, then morale is the way to go, making it easier for the unit to move between captured cities.
There's an analysis with a charismatic leadership above, but sometimes you have a leader than is neither aggressive nor charismatic. Suppose you start with a unit right off the press with 3 or 5 xp, lets assume 5. The promos come at (n^2)+1, so it's 2, 5, 10, 17, 26, 37, 50, 65, 82, 101, etc.
So you attach the leader to the unit, which gives him 25xp. You have 4 initial promos available, and 1 point to the next.
So we have two basic choices: woodsman first or medic first.
Obviously you'd want to do all 3 woodsman promos at once if you're doing that route. And obviously you'd want to get medic III all at once if you do that route.
So we could do woodsman I, II, III, and leadership; Woodsman I, II, III, and combat I; or Combat I, Medic I, II, III. And keep in mind a 5th promotion will be available as soon as that unit kills something. So the last choice could do leadership next or go into the woodsman promos.
There are short-term advantages and disadvantages to each choice. The woodsman without leadership but with combat is the most powerful. Woodsman with leadership starts with no combat promos, but still gets the 2 strikes, which is good against weaker non-mounted units. Woodsmen are good for attacking units in the woods that receive no defensive bonuses, as they get the +50% forest attack anyway. Going woodsman first means your unit can rack up points while your stack defends in the woods, but also risks losing the unit to some extent as they're likely to be the best defender. Going combat and medic first gives you a better healing unit at first.
But I'll confine the rest of the post to whether or not it makes sense to use leadership if your goal is to get an ubermedic (woodsman III, medic III) with the fewest possible points.
Without leadership, you would need combat I, medic I-III, woodsman I-III, for a total of 7 promos, or 50 xp. With leadership, it requires 8, or 65 xp. Subtract the initial points from each: 50-25 = 25, 65-25 = 40. Due to leadership, divide the second one by two: 40/2 = 20. So leadership would require you to effectively get 20 more xp, while without leadership it would require 25xp. In terms of getting to ubermedic first, leadership helps. Leadership is an investment that pays off later in exchange for short-term opportunity costs. In most cases it is worth it, unless giving that extra promotion instead of leadership is game-changing.