It's hot where I am at too (Arizona) and I don't think you can get away from sweaty backs in that kind of weather, but I am a big believer in breathable material. I personally go for high quality wool felt. I have both 5-Star and Impact Gel and like them both equally well. I don't know how much the gel helps on the Impact Gel, I think I basically like it for the quality felt.
I tend to avoid the Professional's Choice Air Ride Pads because the ones I have, which granted are about 10 years old, do not breath well. They are wool on the bottom but the centers are foam and the horses really sweat with them. I don't know if the current Air Rides are the same or not. They are also not as thick as the good wool felt pads.
I've never tried Cashel or Thinline so I can't comment on those. I would guess the Cashel is that neoprene type material like their soft-saddle is made of, and if that is the case it wouldn't breath well either.
I trail ride too, and I will usually put a wool saddle blanket under my wool felt pad to keep it clean. The wool blankets are easier to wash than a saddle pad. The only downside is that they can travel a bit (migrate backwards under your pad) if you climb a lot of hills. But I normally can ride for about 5 hours and never have to make adjustments unless we do a ton of hill climbing.
I think a friend of mine summed it up best. Which would you rather have for your own underwear? A natural, breathable fiber, or hot sticky neoprene/foam? That made a lot of sense to me, so have stuck with the quality wool ever since.
A thick wool felt helps disperse pressure points better than the thinner, denser materials too. I have both 3/4" and 1" felt pads. I use the thinner pad with my wider horse, and the thicker pad with my narrower horse (they use the same saddle). And since I also have a wool blanket on the bottom (a single layer Mayatex) it probably adds at least another 1/4 to 1/2 inch to my overall pad thickness.