I’ll be the first to say that not every horse can go treeless (but treeless may be a better option for some horses), and that a bad treeless saddle will do far, far more damage than a bad treed saddle. Treeless saddles aren’t some magic bullet.
However, I really like Ansurs. They are one of the treeless companies that got it right. My last instructor had and Ansur Classic that she would use on a schoolmaster with huge withers and a bit of a swayback. She had tried tons of treed saddles on him, and none gave him the sort of shoulder mobility that the Ansur did. Now granted, he had some challenging conformation to fit a saddle to. A better conformed (and younger – the sway was due to age) horse may have been served just as well with a treed saddle.
You will need to also invest in the proper saddle pads for the saddle. The pads help create/reinforce a gullet, especially if you go with an Ansur Classic. To not use the proper pads for the saddle is to risk the health of the horse.
As for riding in one: They give you a great sense of your position. If you have problems leaning, you’ll know, because you’ll spin the saddle. This can also be a downside when mounting, or if you need to use a more noticeable seat aid. I have spoken in other threads about my struggles with finding saddles that don’t cause me pain. I’ve found that the Ansur Classic was one of the few saddles that didn’t cause me pain at all. If you have problems with joint pain from riding, it may be a good option for you.
I wouldn’t jump in one. I honestly wouldn’t jump in any treeless saddle. If you like to cross train over fences, then a treeless saddle may not be the best option.
You might be able to get away with using a single Ansur on two horses with disparate measurements. You’ll, again, need to invest in the proper saddle pads to compensate for the different conformations. You may want to have a rep evaluate your situation though. My last instructor used her Ansur on several horses successfully. However, all of them were roughly the same width. The big difference between them was wither height. Using it on a wide horse and a skinny horse may be a different beast. I wouldn’t know.
In short, as far as treeless saddles go, Ansurs are some of the best. If you’re sold on getting a treeless saddle, I think an Ansur would be the way to go. However, not every horse does better in one, and indeed some horses don’t even like them. You may get more bang for your buck from two properly fitted, used treed saddles. If you can contact a rep to at least ride in one, or find someone nearby who has one, do that. They do feel quite different than a treed saddle and that feeling isn’t for everyone.