Its been my experience that leather is harder on the guns finish, and more over all, than kydex, which tends top only wear at specific contact points. Over time, the gun in leather holster usually shows more overall wear and has a worn out, or washed out look to it.
Leather allows dirt and abrasives to be embedded in it, which are not readily removed, if its even possible to do so. Kydex on the other hand, can be rinsed out in the sink under the tap, and instantly dried with a paper towel.
Wet holsters and rust are another big issue with leather. If you live and work in a warm climate, and carry IWB, they are again the wrong choice. Once wet, they dont dry out quickly, and usually takes days of airing. It doesnt take long for a gun in a sweat soaked holster to start to rust, and once it does, you'll be hard pressed to stop its progression.
This old Colt was originally a bright blue. I originally carried it in two Galco Royal Guards. Once summer set in, I needed two, and actually, could have used a third, as both were soon always wet. It only took a couple of months of hot, humid, and sweaty PA summer for the rust to set in heavily, even with daily wipe downs. Once the rust started, it was aggressive, and I had to have it polished out and had the gun refinished in the hard chrome you see now. I also switched to a Blade Tech kydex IWB. Rust dropped to nil, and only at the grip panel on the body side where the sweat guard didnt cover it. The black streaks you see, are actually mirror polished chrome, and not actually "black". Thats also 10 years of hard daily use, and in the same holster. Up until then, I was usually going through a couple of good leather holsters a year. $50 over ten years works out a lot better than $150 a year with leather. While I no longer carry the Commander, I still have both it and the holster, which is a usable as the day I bought it.