The Mules I carry and use are in CPM 4V, K-390, Maxamet, and PMA11. All are non-stainless steels as that is my preference. I find all of these to be excellent performance steels and I would recommend any of them without hesitation, but, as with anything else, blade steel is a personal choice. The climate in which I live and the way I care for my knives does not require corrosion resistance, and I prefer a good tool steel over "stainless" steels.
The one Mule Team knife in a steel I would very much like to experience is Crucible Steel's CPM REX-121. I think this would be an excellent choice for a working knife like the Mule, and others.
But this thread is about sheaths. The leather is attractive, traditional, and nice to have. The Boltaron is tough, practical, and holds securely in a variety of carry positions. I can remove and reinsert my Mule in its Boltaron sheath without a noise issue, same as leather. Either sheath is a good choice so it is a personal preference decision. Good looking traditional, or bomb proof modern. Why not get both?
Here's a link to Crucible's CPM REX-121 in case you've forgotten about this steel. I expect it would be a great choice for a Mule.
https://www.crucible.com/eselector/prod ... pm121.html