As alien, always aim for the human's head. It does much more damage. Hitting the legs does very little damage, and hitting the body does moderate damage. But in general, always, always, always aim for the head. You don't even have to jump with any alien to hit a human's head.
As human, it does not make a difference. Hitting the aliens in the face does the same amount of damage as hitting them in the legs and body. Just practice dodging a lot, to make it harder for the aliens to hit you in the head.