You use rubber and if will just start cracking and absorbing water in no time. I personally had hotchkis springs which they provide delrin for the rear shackles and rubber in the forward eye. No issues and drove fine.
I just took my polyurethane bushings out of the leaf springs because I did not like the bind in the leaf springs. It felt like they would wind and unwind under hard acceleration. It was unpredictable. It also rode like a dump truck. Even with good shocks (Koni). I left the polyurethane in the frame, but it is rubber in the lower shackle and front spring eye. If you go with polyurethane, make sure that your springs are lined up front to rear over the axle. If you have to force anything to get it together, you are preloading the spring. There is no give at all in the front bushing with polyurethane.
I've got the delrin's from Keith in mine and they do make a difference in side to side movement. My rear tires would rub with the new rubber bushings when doing slalom's. After I installed the delrins the rubbing stopped. He has a good video on his website showing the difference.
Just a thought...If your car is a street car with leaf springs, Rubber would work just fine. I'd say think lightly about those who say rubber will fail in no time. Our cars are 30+ years old...still with the rubber bushings. That's not to shabby...and the cheapest way out. Just my $.02