I have both an alpaca roper and a mohair roper cinch (I think they're both weaver but I could be wrong) and I've been VERY happy with both of them. I rinse them out pretty often with just water to keep sweat and hair from building up (maybe once a week, takes about two minutes and let them dry overnight). I ride 3-4 days a week, 3-4 hours at a time in lots of sand and some steep hills, walk trot and lope. They've held up really well with regular rinsing and the occasional good soak and scrub with a blanket wash or very mild shampoo.
I do really recommend going for the solid stainless steel or brass hardware (hard to find). The plated kind tends to rust really fast and cause problems like you are having.
I agree with Joe4d, if that rub mark is your buckle that cinch is just a little small. Go 2-4" longer than the one you have (measure from the very ends of the buckles). With the buckle a little higher on the horses side there will be less pressure on it. Also, make sure that the rings in the middle of the cinch are perfectly centered under your horses midline AFTER you are completely cinched up. This way, the dee's wont rub under his armpits and your buckles will be even on either side. When you first start cinching up, your dee's should fall just to the right of the midline so that once the cinch is fully tightened they are centered.
good luck :)