Deciding between leather, nylon, and Kydex (thermoplastic) holsters comes down to preference. I have personally had great experiences with all three types. One thing to consider is a Kydex holster generally must be molded for your exact gun make/model in order to have good retention. This means you might not be able to find a Kydex holster for your rare gun in a wildcat caliber. Nylon holsters, on the other hand, usually come in size ranges that fit a certain barrel length. Of course, the downside of this is that a thumb break will be needed. Most leather holsters are also made to fit an exact gun make/model, but there are also generic size offerings available. Kydex holsters are also very rigid which can make them less comfortable in some situations.