The bronze body pump is intended for use with fresh water, that is, water that has oxygen in it..fresh from a well or municipal water system. The standard cast iron body pump may corrode when pumping oxygen rich water..like in a DHW system.
However.. as cheap as an iron 007 pump is, and how expensive a Bronze pump is.. i have always opted for the cast iron pump, they are $74.95 NEW at my local lumber yard, and can be bought on ebay for around $50.. The bronze pumps are about $200 pluss.. I have used the cast pumps in oxygen rich water for many many years.. and have only once had a pump grow enough rust scale to cause a problem,, and I fixed the problem in about 15-20 minutes... if you put shut off valves on each side of the pump, repairs or replacement takes only minutes..
So, depending on your application, and whether you want to spend the extra cash, you may want to go with a bronze body or the cast iron..