Kydex doesn't creak. I took my 10 year old daughter to the movies last weekend, I thought everyone in the theater knew what that creaking sound was every time I moved, but not even a curious glance.
Kydex releases the gun more quickly. I guess this depends on the holster, but once it releases the gun there's no more drag. Leather seems to drag until the front sight is clear.
Leather is more comfortable for me.I spend most of my day driving, Kydex doesn't give, period.