Merkin

hmm if there are stiffness differences, would the bushings on the softer units last longer?

diagnosticator1

can be varied. Besides the rubber compound used, the cross sectional profile of the rubber insert effects the stiffness or compliance a lot, and depending on the configuration, can be stiff in one direction while allowing for lots of compliance in the other directions. Considered from the aspect of the durometer value only, my guess is that softer material won't last as long, but it's not a simple relationship, the conditions of service effect the durability considerably.

Merkin

and leave these crappy rubber bushings behind.

diagnosticator1

However, the main reason the OE parts use the rubber pivot bushings is due to the road noise/vibration/harshness isolation the rubber parts provide. While spherical bearings are precise for holding relative positions between parts, the NVH that would be associated with the spherical pivot bearings will be considerable greater. The NVH transmission into the cabin would be unacceptable and tolerated by only those who demand the benefits despite the higher NVH.

BTW, there are ways to effectively protect spherical bearings from road dirt and water contamination besides enclosure in a rubber boot.
Another option is to make a set of custom boots using castable resins.