Would like help on choosing a new sump pump

I would like to replace my sump pump, as it is going on 16 years old. I currently have a 1/3 hp, for a 3000 sq ft house. I was looking at the Zoeller M53, but also saw the M76, which is made from a thermo plastic, as opposed to cast iron. The M76 is from Zoellers AquaMate line. Anyone have any thoughts on these two choices? Thanks!