In general, glass is less durable than plastic, except with regards to scratching. This is due to the fact glass is harder and more brittle than plastic. Without getting into the details and minutiae (i.e. I know there have been cracks in the plastic of previous iPhones), I just want to point out one thing.

With the introduction of the iPhone 4, Apple is for the first time offering their own form of iPhone protection. They call it the Bumper.