Stamped steel impellers may be a little more efficient, but I think cast-iron impellers tend to last longer. I have seen photos of steel impellers rusted away or sheared from the hub.

Re the comment on fewer blades in a water pump for a factory AC car, that is true with 60-70's small block Mopar engines. It is because AC cars have a different pulley setup, w/ a smaller pulley on the water pump so it turns faster. I haven't seen any variation in the 82-85 300D & 300SD engines. All cars seemed to come w/ the same options - AC, pwr steering, sunroof, ...