Welcome to the Yard CloaknDagger, please feel free to stay and chat for a while.
In reguards to your two questions. Leather, kydex, or nylon, it depends so much on what you are doing with the knife, safe queen, dive knife, EDC, hiker, BOB knife.
The other is Folders, Benchmade 350, Spyderco Endura II, Spyderco Rescue. Swith and Wesson SWAT, Cold Steel 4" tanto Voyager, and old school Leatherman Supertool (if that one counts).