The reason why the metal ones don't fail as much as the plasic ones is because the back part of the water pump falls apart internally. This metal ring that falls out puts pressure against the impeller causing it to crack and fail. Using a metal one and it shouldn't fail. But the back part will most likely fall apart.
I've had three water pumps that 'fell' apart since I've owened the car, none of them acutally broke but there was serious wear and trear to the impeller because of that metal ring coming apart. My susggestion is to buy a new water pump from Ford because it includes a new housing.
Or you can buy a new metal pump like everyone else
