hello, done my old 1.9tdi pd bora and my 1.8t lc with water pumps from euro car parts and they only supply water pumps with a metal impeller. hope this helps
 
i asked the same question and got told its because they are from different manufactures, not sure if its a difference in quality or just the name on the box to be honest, i'd say the best thing to do is to give them a ring and ask them direct