Good question. Here are my thoughts.
If you can have both, then you should. If you can only have one or the other, then I would weight it up like this. If both are going to say good things about you, then pick the one higher in status. If one is going to say much better things about you than the other, then pick that person.
If you think that can get a somewhat better recommendation from the assistant professor than from the head of the department, then that is a bit of a quandary. As status of a student's referee and their level of supportiveness both affect acceptance decisions to uncertain degrees, you will probably just have to make a call based on your own intuition.