Not all polyurethane bushings are created equal, but most are harder than rubber. I went with Polygraphite from P-S-T on my C4 suspension after reading it was less prone to squeaking. Replacement rubber bushings are NOT available for C4 Corvette applications so polyurethane is your only choice unless you're willing to go with even harder materials like Delrin.
Here's some good reading:
http://powerflexusa.com/howtochoose.aspx
http://www.spohn.net/blog/2010/04/26...hane-bushings/
https://www.eeuroparts.com/blog/2120...poly-bushings/
http://www.superchevy.com/how-to/cha...your-corvette/
OEM Rubber
Factory bushings are made of rubber, a natural material harvested from certain kinds of tropical trees. Rubber bushings are soft, around a 60-65 Shore A rating, and they flex and rebound as your chassis and suspension moves, exhibiting very little noise, vibration, or harshness (NVH) in the process.
However, as mentioned in the intro, those same properties make your Corvette less responsive during serious performance driving or racing. And don’t forget that rubber is biodegradable and will break down over time.
Polyurethane
Unlike natural rubber, polyurethane is a man-made plastic. This material is both elastic and strong, and can be formulated to be very soft, extremely hard, or anywhere in between. Polyurethane bushing hardness usually ranges from just slightly harder than rubber (around 70-80 Shore A rating), to firm (around 80-90 Shore A rating), to very firm (around 95 Shore A to the harder Shore D rating). And unlike rubber, it’s immune to age- and contaminant-related breakdown.
One of polyurethane’s biggest “drawbacks” is its bad reputation from decades ago. Back then, poly bushings were in their infancy: they were too hard and too harsh-riding, and they squeaked incessantly. Thankfully, today’s polyurethane is much more advanced: many street poly bushings are only marginally harder than rubber, but retain poly’s quicker response, higher load-bearing capacity, better tear strength, and stronger abrasion and contaminant resistance. Some companies offer graphite-impregnated, self-lubricated poly bushings too, but in most cases, when regular poly bushings are installed and maintained correctly, they don’t squeak anyway.