i was recently looking for a new helmet and a friend of mine, who is pretty fucking knowledgeable on all things bikes, explained to me the difference between plastic/polycarbonate helmets and fiberglass/kevlar/aramid helmets.. he has tested both plastic and the fiberglass and he swears by the fiberglass (which you will also note are more money but I guess depends on how much you think your head is worth? Do you have a $150 head or a $500 head?).
http://en.wikipedia.org/wiki/Motorcycle_helmet