There are ups and downs of both. Polysulfone isn't nearly as sensitive to ambient temperature as stainless steel so you don't have to worry about losing heating as hot liquid is flowing through on a cold day. The real difference between the two comes at the threads. The threads on the inlet and outlet are not difficult to cross thread with the polysulfone heads and once you do you are screwed. Stainless can be cross threaded too but it's not nearly as easy. One other thing is I have a lot of heavy parts on top of my pumps and that head is basically supporting all the weight - so it was better for it to be stainless in this case. It just depends on your application.
Well I finally got around to rewiring my old pump and it indeed does work. Sorry it took so long to get back with you. Will you do $20 shipped for one of the heads, to 21788? Also do you still have any ball valves left?