I prefer nylon but that's because the leather is to hard for me to tighten up. That being said. Nylon is more likely to slip then leather if you don't get it tightened good. As for curb straps I use a curb chain with the leather end pieces because that's what we had.
Has anyone used the Weaver Smart Cinch? If so what's your opinion of them and are they worth the cost?
I think it helps a little. I have a hard time cinching as well so any little bit helps.
I have leather on my saddles now, but I prefer nylon latigos. The nylon are much easier to cinch IMO
As far as curb strap I use either a double chain or a leather. I do prefer chain but it will rust over time. Wider is better no matter what you use.
I agree about the nylon latigos. Leather ones look better, but if they get wet or sweaty, or even if they get a bit old, they tend to stick together and make it pretty hard to smoothly tighten your cinch. I hate having to yank the latigo to get it up to the next hole because it's stuck to itself.
As for curb straps, I use thinner full leather on my snaffles to prevent them being pulled through the mouth. On my curbs, my strap of choice is this
With either nylon or leather ends, depending on what's cheaper. Vida is right, whatever you choose, the wider the better. I prefer the double flat link chain just because that is that much less that I have to oil, plus it's hard to find full leather ones that wide.