I've never had a Thornhill dressage saddle, but I did have a Thornhill A/P saddle. Specifically, I had the Thornhill Jorge Canaves Trail saddle. Outside of a custom saddle, it was the only one that fit my mutton-withered, broad-back mare. I had that saddle for several years. It was very comfortable and held up quite nicely. I only sold it to get a more "traditional" looking saddle since I was getting into showing. I now have a much more expesnsive custom Albion. In summary, I would not hesitate to buy another Thornhill. They seem to fit a wide range of horses and are a good value. My old Thornhill is now owned by a friend of mine.
I currently ride in the Germania (sp?). I love the deep seat and the blocks are nice. The back of the saddle is wide like many Albion saddles that I have tried, and is well padded. So far the saddle fits my wide draft with withers and a more narrow high withered warm blood that I'm riding currently. The saddle is heavy and the leather is slick you can tell that it is an relatively inexpensive saddle. However it have been a great workhorse for me and I will only sell it if I find a Wolfgang solo in a nice price range. Also look on ebay I got my Thornhill for a steal.