I started IWB carry before Kydex was even a thing, so it was always leather for me. When Kydex really started taking off, I tried a couple of them, but always went back to leather. Kydex is too hard, causing too many pressure points as you move around, while leather, being soft. will flex and bend with your body, so no pressure points.
Two IWB holsters I have liked and carried with extensively are Milt Sparks, either their VM II or their Executive Carry, and El Paso Saddlery's C-Force line.
Get a good stiff belt designed for concealed carry holsters. I like Galco's SB series. Been using them for 30 years with no complaints.
The guns I've carried comfortably IWB since 1980 have been full sized, all steel, 1911s, K-Frame S&W revolvers, Glocks (17, 19, 26), a Ruger mid frame revolver (Speed Six), Sig service-sized pistols, and an assortment of smaller auto pistols and revolvers.
