[rick3000]

A failing water pump will most likely make contact with the engine case, not just the WP housing. Just look at a picture of the WP, it's a giant spinning blade directly facing the engine casing.

Here are the basics of the metal vs plastic impeller argument:
-With a plastic impeller, if it fails, the plastic makes contact with the engine case and tends to break off in chucks, which normally get trapped in the radiators or oil cooler (not ideal). Since it is plastic vs metal, their is essentially no damage to the engine casing.
-With a metal impeller, you get fine metal particles that circulate through the system freely, and hole bored into the side of the engine.