Kydex all around, but especially if you have the holster against bare skin. Leather is brutal there.
Leather is bad enough dry, but once damp with sweat, its abrasive as hell. And some of the chemicals/treatments they use on them just add to the burn.
Kydex doesnt really rub you much at all, if any. When damp, it just slides across the wet skin.
Of course, both are better with a tee shirt between you and the holster. But if your the least bit active, sooner or later, that tee will ride up.
And thats just "your" half of the equation, you dont generally get rusty guns with kydex holsters. Leather on the other hand....