Depends on what you value and how you will use the kit.
Leather looks good, feels good, doesn't screw up your mirror polish unless you live in sand-land where leather will collect that sand and rub your blade. But leather is soft, it absorbs water, and will show wear. Not to mention the material is expensive.
Kydex has its own properties and issues. It's lightweight, doesn't absorb water, doesn't show wear, and the material is cheap. Retention doesn't require a strap. But it looks cheap compared to leather, attachment to belt/gear can be easier or more difficult depending on what style you prefer. Another "benefit" of kydex, which is more prescient for pistols.... is that it doesn't slowly wear out and become dangerous like leather will. A kydex holster is either serviceable or not. A leather holster can wear out, still technically function yet be dangerous to keep using.
I'm a kydex guy, but I dont have any show-pieces that I need sheaths for. I don't dress up and display a knife (I conceal it, along with a pistol).
I love leather, but for how I carry knives and what I want from a sheath, kydex's properties are superior.
Prefer what you want, but I don't own a single leather sheath or holster. Everything is kydex or polymer. It just doesn't work better for me in any way besides looking and feeling awesome.