The four BOBs I have built all have leatherman in leather cases. There is room in the case for the extra bits and a sharpener. I built tough phone coil-cord lanyards for each one and prefer this to the belt clip. If you're stuck somewhere you don't want to loose that important tool. A coil cord lanyard will allow you to use the tool without setting it somewhere and forgetting it.

I'm not answereing your initial question. I prefer leather but only because it's been used forever and I'm more familiar with it than other materials. I would assume, any material that has been deemed mil tech is pretty good stuff. And the material need for BOBs and EDCs may differ.