Not sure if you are going for a cartoon or realistic looking head - but I would definitely concur that working from a front and side reference picture will help.

At the moment the large eyes and reduced mouth area is making your model look like a child, which again, might be what you are going for. At the same time, your model is looking pretty gaunt and wasted, which is contrasting against the child like qualities in an interesting way. Of course, this is just my opinion.

I don't know much about NURBS - I tried working with them for a month or so without much luck, but I have heard a number of people on this site recommend against them.