Sometimes systems or components for a given automobile are designed and built by an outside supplier. Some cars, like Chrysler products, IIRC are about 50% outsourced.
Maybe this was the case for the Duratec and Zetec, explaining the difference in impeller materials.
I am an engineer and have extensive knowledge and working experience with centrifugal pumps. In the chemical and refinery industries you rarely find polymer (plastic) pumps and usually only where corrosion is a great problem (from acid) and the temperature is low.
Granted, the applications are different, but there is a great physical similarity between the design of the Duratec pump and chemical process pumps.
The metal impeller has a higher rotating mass, but this is probably neglible.
I would choose the metal one any day!
