I am a fan of kydex. I think it holds it shape really well, does not collapse when you unholster and go to reholster. It is also thinner so can provide additonal comfort for IWB holsters. I carry a hybrid holster, kydex shell with leather backing worn IWB. My husband made it for me. I like it, I honestly think kydex looks nicer than some of the all leather holsters I have seen. I say that mostly because all leather just make me think of bulky holsters.
Kydex should not scratch your gun, sometimes dirt or grit can get between the slide and kydex. Leather can wear down finishes also so I don't think one over the other is better for that.