kydex is nice because you have many more options for mounting it either on your person on on a pack and it is easy to clean out and doesn't retain any moisture. it also is very light.
leather looks better and is not nearly as rough on blades with coating, but it is a bad idea to bring it into a wet or high humidity area because it will retain moisture and eventually corrode your blade.
IMO for edc carrying a fixed blade, leather; for field use, kydex.