Agree with the ride part; not so sure of the stability statement. A SIB has both tubes touching the water at rest, so is reasonably stable. A RIB may or may not have both tubes in the water at rest (depends on design), so actually may be less stable.
As far as material, I prefer hypalon, but that's a judgement call. The late model Zodiacs seem to hold up pretty well. PVC is lighter, too, I think, for a given size.
I had a 14' Achilles SIB for about 5 years, and had no complaints from a workmanship standpoint. Well built boat.