I've got kydex and concealex holsters, and comparing them side by side, in this instance the thickness are so near the same as to be indiscernible to the naked eye. However, there are notable differences between the two materials, in both texture and color.
The Concealex looks blacker (in a basic black sheath/holster). Also, in the samples I have, the Concealex has more surface texture on the outside, making for better positioning of the holster IWB (I have heard people complain about plastic holsters moving around while being carried IWB). In retention, I can discern no difference between the two.
I have one of Mike Sastre's Moran sheaths, which I believe is Concealex. I also have the first incarnation of the Nimravus Cub sheath (which I believe is Kydex, vs. the current sheath, which I think is Concealex). Both work extremely well, although I have to give the nod for workmanship (as well as design and usability) to River City (Mike Sastre).
oops, forgot to mention one other thing. Kydex is only available in basic colors (black, maybe grey, olive... maybe). Concealex is available with different colorful patterns, including, IIRC, Advantage (type) camo, basic camo, desert camo, urban camo, and the neat carbon fiber pattern as available in the BladeForums Native neck sheaths.
------------------
...when it absolutely, positively has to be cut right now...
Outlaw_Dogboy
[This message has been edited by Outlaw_Dogboy (edited 01 November 1999).]