What is the difference between a no-hub coupling (corrugated metal sleeve):
Versus a shielded no-hub (smooth metal sleeve):
Then there is a flexible coupling, which oddly is not called any kind of "no-hub" even though the connection style is exactly the same, but without metal sleeving, hence flexible:
There is a significant price difference between the no-hub and shielded no-hub, with the latter more than double in price. Odd, since I would have thought corrugated metal should be stronger and also cost more due to the required processing.
Most of all, how are no-hubs and shielded no-hubs different?