I have the Zero and Heavy belts from Tenicor. @Mack recently sent me some
Specialist Rigid belts and
Pillow-Wedges to T&E.
I've been meaning to write something, so here's my take:
Zero: Nylon strap with cloth sewed over it. The Zero buckle is excellent, but otherwise this belt similar to any non-firearms webbing belt you can buy.
Heavy: Thick double nylon strap. As for the Zero, the buckle is excellent. Good compromise between stiffness and flexibility. Softens over time, and not in a good way.
Specialist Rigid: Nylon with thin polymer insert. Uses a standard flat metal "3-bar slide" as a buckle. Similar to the Zero, I can slip the free end of the belt under the buckle. The belt itself is the best I've used for AIWB. It's not noticeably thicker than a simple strap, but the polymer insert provides vertical stiffness to support the holster and prevent crumpling. It's stiffer vertically than the Heavy. Because the belt and insert are so thin, the belt is nearly as flexible as the Zero in the horizontal direction. It conforms to my body and holster, flexes when I move or sit, and supports gun, mag carrier, and clinch pick without having to tighten the belt more than I would to just keep my pants from falling down. And all that for $33, about half the price of the Tenicor products.
Sizing: I wear 31" pants, and use a 32" Specialist belt. This leaves ~3" of free end, just enough to adjust for gear and after meals.