[starbuster]

I've personally only used leather. I prefer it for almost all of my tack. However, I've felt biothane that was very similar to leather, so I'd probably pick that over nylon if I had to choose.

If you don't mind buying used and like leather, I see nice rear billet and back cinch sets on eBay all the time, though I can't recall the prices (as I haven't been looking for them, but have scrolled past several). I'm sure they would be cheaper than new, though I'm not sure about biothane or nylon.

 

[Elessar]

Your poll doesn't allow for other materials like mohair or fiber. I've purchase an 36" mohair cinch and used it for a rear girth that is soft, easy to clean and light weight.

 

[QHriderKE]

For a rear cinch, if I didn't want leather I would probably get a mohair one that's woven a lot like a front cinch.

As long as the leather is sound (no serious signs of cracking or stitching coming loose), you can give it a light scrub with a soft brush, saddle soap and water before applying a light coat of neatsfoot oil.

If your saddle is leather, I think nylon or biothane would look unmatched and tacky.

 

[marymane]

I don't think Beta Biothane wouldn't not work for a back cinch as a material but I've only come across one and the issue, as with the nylons, is that they are very straight and quite thin for back cinches and barely suitable for the simple purpose of ensuring the saddle stays down and put in hilly terrain because the pressure would be so localized. If you're going to rope? Forget it. The nylons are cheap and not designed for that purpose at all. I wouldn't trust the hardware on them either. A lot of nylon products of that sort are nylon because they're being marketed for people who want to pay bottom dollar and it shows in the quality. There are some things you can get away using synthetic/cheaper materials for and some things you can't.

The horse's belly area is pretty sensitive so I can easily see the nylon material also causing rubs would be an issue.