Depends on the application. Poly (read: hard plastic) is much more prone to scratches but the chances of shattering the thing are quite low. In fact, if you've endured a big enough impact to wreck a plastic windshield, you will likely have much bigger problems that require your attention. A poly unit can also be repaired fairly cheaply if it gets hit by a fast-moving rock or other large debris. Of course, none of these accessories mean you can leave your helmet at home.