The polyurethane bushings (Prothane or Energy Suspension) are a harder material than the rubber. There is slightly less deflection than rubber, they will ride a little harder than rubber, but on A arm bushings that won't be very noticeable.
A rubber bushing is vulcanized into the metal sleeve, so it must be torqued while the vehicle is set at ride height. It also has a slight spring rate, as the bushing rotates, the rubber is stretched slightly.
On a poly bushing the outer sleeve, poly isolator, and inner sleeve are just pressed into each other so the bushing has no spring rate and can be torqued when not at ride height. The plastic must be lubricated with the nasty white grease that comes with the kit or it will squeak like crazy.