If you are using PEX pipe for infloor heating... I will assume that it is going on an "open system". as such, non-ferrous pumps must be used, so Bronze is the way to go.
If this is on a closed system. Wrong pipe to use. it will allow oxygen to permeate into the system, causing several issues including "rusting" all ferrous components.
I'd use PEX with OB (oxygen barrier). this allows you to go either way (open or closed) and allows for future installation of proper "Closed" system.
Only use Cast Pumps on closed systems.
I have a standard 50 gal water heater in the loop as a buffer/volume and I have HePex poured in the floor. I am using a TAKAGI tankless boiler/heater (190kbtu max) with no sidearm domestic hot water unit. I guess I can use the cast iron pump according to your reply.