So if you start searching for holster online or in store sit will often come down to leather or kydex in the high quality holster arena. Leather for the longest time was the standard for holsters. Leather is high quality for sure, but is a bit obsolete. Kydex is a more modern polymer design that is resistant to the environment, and as well as more damage proof.
Kydex is now the most common holster among police and military. Leather does conform to the body and is often considered more comfortable to deep concealment carry, but as a duty holster it is more easily damaged and deformed. In my opinion (And the police and Marine Corps) kydex is the way to go for long lasting duty use. However if one chooses to use leather I do suggest the purchase of leather protectant to help extend the life of your holster.