I’ve used both leather and synthetic gear over the years. Leather for my classic guns and casual carry, kydex for serious defensive use. Why limit yourself to one material?
Because I'm old and set in my ways, besides I just plain don't like the looks of the synthetic materials in slings and holsters. So for me it's leather.
I'm old, too, and have been a fan of leather holsters and slings. I do have a couple of kydex holsters as well as a few Crossbreed's that mix the two which a hard not to love. I still vote leather but sport kydex when the occasion calls.
Most shooters I know use both without thinking too hard about it. I am also included. Leather feels nice and looks good, but synthetic is better when it's raining or you're really putting it through its paces. Use what works for the job and don't worry too much about it.