If they aren't EPDM, ethylene propylene diene monomer, then they are usually CR, polychloroprene or chloroprene rubber. This is why some manufacturers call their CR based belts a POLY belt, because they are polychloroprene.
There is other factors other than wear length, aka how long they last. Some racers swear by CR Poly belts because they won't stretch as fast under extreme heat like the softer EPDM belts will. Dont believe me then thats fine, but do some reading on racing forums. I have a friend who is a dirt track racer, want an argument about CR vs EPDM then grab a beer and mention it to him, its gonna take a while.. LOL
Of course there is many other variants, some are fiberglass weave, you can even get carbon fiber woven belts if your pocketbook is fat. Either way, EPDM or CR, the vehicle manufacturers still give pretty much the same change interval. So it doesn't matter. I just changed my tensioner and belt, in another 20-30k i will toss on a new belt, and in another 60k i will replace the tensioner again! Cheap insurance...