it can be both. for binding i would suggest you bind to the polygon for the head, and then converting to subd with the the proxy option. so the binding to the poly will still work, and the subdiv will follow. you just need to make the poly not renderable and you are done ...

edit: oh if you bind more than one polygone to the same skeleton, just make sure that the touching verts have the same weight - so the character does not get holes when animating - of have the neck be longer so you dont see holes when you look in the shirt ... hope that made some sense