One thing no one has mentioned is the way the hardshell is attached to the foam of the helmet. On cheaper helmets, the hardshell is typically taped or glued on; on more expensive helmets, its bonded to the foam. I have no idea if this provides any more protection in a crash, but I find that bonded shell helmets tend to last a bit longer. I've had helmets where the taped-on shell begins to peel, or cracks at the edges.