Hope you get the specialized you saw advertised, as that one seems to be your first choice!
Personally, my treeless is a Torsion, which I have had good success fitting to a variety of horses without having back soreness issues (*knock on wood*).
I have ridden in a couple Bob Marshall Sports Saddles and find they do weird things with my center of gravity.. they put my leg way out in front of me, forcing me into a chair seat (and seeing as I tend to be that way on my own, I sure don't need any additional help!
). But I certainly know several people who love theirs.