ABS is a type of plastic, Kydex is a brand name. Kydex is a proprietary PVC/Acrylic alloy which exhibits many characteristics that make it superior to ABS for many applications. It has better resistance to chemicals, better heat stability (both are heat formable), superior stiffness, MUCH greater impact resistance and less tendency to warp. It also is somewhat lighter than ABS sheet. The biggest problems with Kydex are that it is not very UV stable (under direct exposure) and it is somewhat expensive.

If you have used ABS for "much longer" than anyone has been selling Kydex, you must be pretty old! - Rohm & Haas Chemical began marketing Kydex in the early 1960's! It is now manufactured exclusively by Kleerdex Company at plants in Bloomsburg, PA and Reno, NV - I've worked at the Bloomsburg facility for just a bit over 7 years now. I repair everything in my house with Kydex - of course, they sell us "scrap" material for a fraction of what you would pay at a distributer.