I'm no expert at this, but I have read a number of articles in motorcycle magazines. This question (hardshell vs. softshell) comes up in that arena as well. The hard shell helmets are supposedly better for really hard hits; however, they say you are actually better off with a soft helmet in lower speed hits.
So while I have given up my tried and true ProTec in favor of a Grateful Head helmet, I would say that a properly fitted ProTec is vastly superior to a hard helmet with insufficient padding or poor fit. If you need to diss on a helmet, pick on the Ace. Remember those?