I have Dales and Fell ponies. Having tried various treed saddles, which even after having them fitted, slipped forward, backwards, or just did not sit right, or suit us, the rider. The only ones that I was happy with were the Thorowgood, and also the Wintec with the interchangeable gullet, and an xx wide one worked best.
I then tried a Trekker treeless, and loved it so much. Ponies moved so much better, and I found it nice to ride in. My husband rides in a treeless Torsion, which he likes. I have had a saddle fitter check my ponies, and also the saddles and she said, used with the treeless saddle pad, the ones I am using are fine, although she would not say the same about all the treeless saddles.
My husband had riding lessons, where he rode in a treed saddle, which he found a bit hard. The instructor was most impressed with his seat when riding, and this is down to the position the treeless saddle puts you in.
I do have a Wintec for posh shows, but given the choice, would ride in my Trekker.....my original one was stolen along with a lot of other things, and the loss of my Trekker saddle hit me badly