It is my understanding that Delrin is a harder material than Polyurethane. Delrin is also less likely to distort which will help keep the rear wheels planted and the alignment from drifting out of spec. I have the FTBR bushings in the IRS for six years, everything still feels solid with no binding. I know some people that went with the Polyurethane kit and they are happy with that setup too. You will experience some NVH with either setup, nothing serious but noticeable. One thing for sure, if the car has a lot of miles on it, the rubber bushing are probably shot.
i did a combo of both. used FTBR in the arms themselves and poly in the subframe and diff mount. i may eventually switch over to delrin in the diff mount. i am very happy with my set up so far. noticeably better handling and not alot of added noise