On a satin blade kydex can scratch the finish...other than that consideration both work well...kydex tends to give a quicker draw if that is an issue....interestingly in extreme cold leather is more durable... if kydex is exposed to the elements in seriously low temps it can shatter...never happened to me but I checked it out and it can happen. My preference for military use knives is kydex....no amount of oils etc keep leather right in a tropical jungle...not if your there a while...but for concealed use involving shoulder carry or a Dundee across the back set up for larger knives I like leather...I also like leather or canvas or nylon for an inner forearm carry...it is much more quiet...infact leather without "snap" straps is ideal for certain military uses... I have sheaths of both...the only thing I don't like is velcro...it is very loud with it's "tearing" noise.