I prefer leather, but I have a kydex holster as well (Fobus model, for a Bulgie Makarov). I do like kydex's properties (slick, fast & easy draw, tough), but I'm pretty anal about my firearm's finishes, and kydex holds too much debris right on the surface where it can scratch the finish.
Most of my holsters are leather. I just prefer the feel, smell, and slight give leather brings. It molds to your body, where kydex makes your body mold to IT (much less comfortable, to say the least). I'd never use kydex for IWB - I don't use IWB holsters much anyway (I have a very bad back, and IWB puts me in even more pain, but I do use the method when necessary), I much prefer OWB in a pancake.
To me, concealment is not so much about the holster (but the right design DOES help a lot), but in how you dress. With all the hype about vests, etc., I've never seen anyone use this mode of dress - much less for concealing a firearm. In the wooly wilds of west central Florida, anyone wearing a photog vest would be looked at as an idiot, frankly. It's too darn hot here 90% of the year for something like that, no matter how much high-tech fabric and mesh is used. I conceal fine in an untucked polo shirt (and I'm 6'1", 135 lbs w/ a commander-sized 1911!), or a T shirt and a open, button-up shirt as cover. This will get you through 90% of casual situations. I carry the Makarov if I have to go with a tucked shirt, either in a tuckable IWB or in a khaki's pocket, in a pocket holster. It's a big large for this, but my khaki's have big, deep pockets.
I do recognise the newer materials' utility and properties, but I still feel leather is still one of the best materials ever discovered for many applications. And, for IWB, I'd rather have something that "gives" a little than a piece of hard plastic in my pants!
Just my opinion, anyway...