Correct, the cast one is your old pump. The stamped impellers look like they were made from a flat piece of sheet metal, and bent into the proper shape. I was looking at your picture, and I noticed that the cast impellers have six vanes and the stamped ones have eight. Also, the cast one looks to have a significantly larger diameter. I would assume these two things would allow the cast impellers to take a bigger "bite" of coolant as it spins. Again, whether or not you will notice any difference in performance between the two, I don't know.
