I've used both. Kydex will wear the finish on the edges of the gun. In my experience leather will last longer. Both leather and kydex will hold a gun securely. I now only use leather. I don't find any difference in concealment, but I only carry IWB.

It's a matter of personal preference. But if leather lasts longer, looks better and doesn't wear a gun as much, why go with plastic?

Bill