Hmmmm.... Frankly I don't know. I tried size 18 every time and they all felt about the same to me (I tried 3 dressage saddles, A/P, and 1 or 2 CC). On other side my Collegiate A/P is 17 and about same size as my Vienna.
Go figure! What I know for sure though you have to be careful about the tree size. 34 size Klasse was VERY wide on my horses while 34 size Vienna was pretty good (I ended up with 32 size Vienna). Also keep in mind that Thornhill is one of those saddles designed for wider horses. I don't think it's the best choice for light-build TB (although I can be wrong
). Not saying you have TB or light horse, just making a comment!