ok, some advice:
1) try poly-by-poly head modelling, youre gonna get alot more detail and a better looking nose with poly-by-poly modelling, you can do this using front and left side ref pics
2) look real good at ref pics when ur doing the head modelling, the head is something that is part of the identity of a person, sure, you can see at the his hat-thingy that hes link, but take that off and you would have no idea of who it is, when you do poly-by-poly modelling you can put in the details (cheeks, mouth, nose, eyes etc) + when you do poly-by-poly modelling you get alot less polies than with spehre shaping (dunno, thats what i concluded .. a18s head had bout 400 polys when i tried sphere shaping and has 290 polies when i did poly by poly
greetz