I ought to learn not to comment on threads like this, but I can't resist.
I have to agree 100% with Saddlebag. There is no possible way in the world, no, not any at all, that any neoprene, or nylon, or fleece covered, or any other solid type cinch can be cooler on a horse than a good old mohair string cinch. Unless the cinch itself is air-conditioned, it cannot possibly be cooler. Mohair string cinches not only wick moisture away from the horse, but they also allow the sweat to dry, thereby doing exactly what the sweat is supposed to do, cool the horse! Neoprene allows the moisture to pass through, but holds the horse's body heat in. That's what neoprene is designed to do. Cordura and canvas cinches "breathe", but when they get wet, the threads swell, making the material water-tight. When wet they don't breathe. Wool fleece-lined cinches don't wick the sweat and tend to hold heat in, but they do breathe a little.
Any cinch with padding, like neoprene cinches, or wide cinches, require the cinch to be tighter to give the same holding pressure as a narrower string cinch (because it spreads the pressure over a larger area), which is contrary to what one would think. A wide cinch will often cause rub sores on the backs of the elbows of a horse, particularly with a 7/8 or full-double rigged saddle. Ropers use wider cinchas to hold the saddle in place with the pressure of a steer stopping hard at the end of the rope attached to the saddle horn, and they really crank the saddle down tight. But, they don't do that when they're out pleasure riding.
To those of you who indicate your horse does better with a wide or otherwise specialty cinch, I would put forward something for your consideration. Your horses most likely like the wide cinch better, because you are not cinching it as tightly, relative to cinch area, as you would a narrow cinch. Since the pressure is spread over a larger area, you would have to cinch it tighter to get the same amount of holding pressure you would with a narrow cinch at a lighter pressure. The idea I'm putting forth is simply that if you weren't over-tightening the narrow string cinch, your horse would like it just as much. Many people over-tighten their cinches needlessly. That's why their horses get "cinchy". If you're riding a bronc, that's one thing, but a steady, well-broke horse on a trail ride doesn't need a very tight cinch. Snug is sufficient.
To demonstrate what I'm talking about, ask yourself whether you would get better traction from a dual-wheel pickup or a single-wheel pickup. Common sense would tell you the dual-wheel pickup would have better traction, but that's wrong. The dual-rear-wheel pickup has such light loading on each rear wheel (like the wider cinch) that you can get stuck on wet grass, whereas the single-rear-wheel pickup does just fine.