Boils down to personal preference. Very personal preference, if you think about it -- after all, the thing is going to be underneath your outer clothes with you, and that's pretty darn personal.
Me, I like Kydex. I live in the Pacific Northwest, where there's frequently a good chance of getting soaked, so I like the reassurance of having a holster that's as waterproof as it can possibly get (and I put a Glock in it too, another waterproofed option).
I like the positive retention offered by good Kydex and the solidly reassuring "snick" as the gun moves into place.
I like the slightly faster draw of Kydex.
I've never found mine uncomfortable, but I'm told by people who know that leather is more comfy. *shrug* I wear mine every hour I'm dressed, and rarely notice it's there. But again, that's an individual thing.
Oh, and I also like the idea that if someone glimpses a plastic doohickey affixed to my waist, they are more likely to think "cell phone" than they are if they spot the same doohickey in leather. Concealed is concealed, but stuff happens.
pax