I'm not much help on the subject. I like both for different purposes. Lately I've been swapping between IWB and OWB for my EDC....and therein lies my switch between leather and kydex.
On a belt, OWB, it's leather all the way -- at least for daily carry. My competition rigs are all kydex, but that's a different story.
For IWB I want a stiff-topped holster. I use a CompTac MTAC for my 1911's and a Sparks Summer Special for my Glocks. I could be convinced to go to the SS for a 1911 as well, but I use the tuckable feature when I dress for the office, and I didn't have to wait a year to get it.
I love the stiffness of the kydex for IWB, but I've got a couple of "pure" kydex IWB rigs (like the Infidel), and they're not so comfortable against my side. I prefer leather against my body to kydex. Guess I'm old school too.
Kydex advantages: Doesn't "creak" when you move, doesn't require a break-in period, high-quality kydex is cheaper than high-quality leather, and to answer the "tactical" question, yes, it *is* faster on the draw--at least for me--proven on a timer...to the order of a couple of tenths of a second (not significant in the real world), and it doesn't wear out.
Kydex disadvantages: Doesn't "give" at all (actually, this is both advantage and disadvantage), noisy draw compared to leather, clips/attachments aren't as flexible as leather (but can be modified to fit your belt, etc. with a hair dryer).
Leather advantages: has some "give" to it as you move around. Generally more aesthetically pleasing (personal opinion), quieter draw.
Leather disadvantages: cost, wears out eventually (can get soft/floppy in some models), requires the same kind of maintenance as a good pair of shoes, must break in to the gun in many designs.
Another question I hear about Leather vs. kydex all the time is holster wear. Since I use both, I'll tell you my experience: Both wear the finish...they just do it differently. In the long term, a carried gun gets holster wear...consider it "character."
I've probably only clouded the question for you. I'm one of those guys with a box full of holsters, and I've come to the conclusion that there's no one "perfect" holster--just holsters that are "perfect" for a given purpose.