It depends.

Kydex and its variants are easily cleaned if wet or dusty/greasy. Some Kydex-like products are even friendly to gun finishes. Kydex, being heat set, has to have a little extra care in really hot environments. It won't stretch or shrink.

Good leather is gentle on gun finishes and is more comfortable for IWB carry. It does not like getting really wet and is difficult to clean at times if nasty products get splashed on it. Due to its ability to change shape, it works well with pancake style holsters.

Both have the ability to retain firearms well as well as, in a properly designed holster, producing a quick, safe firearm presentation.