See, I ran into an issue with a deep seat on my short backed uphill PRE posted above. The deep seat put too much weight at the base of his long withers. Partially because I had less room in it and it tipped my weight a bit too far forward, and a deep seat doesn’t work for my shape (thanks to years of weight lifting and squats, I’ve got a booty).
So I went for the same size (17.5) in a slightly more open seat with more of a flat spot to it. My balance is perfect now. The panels are slightly upswept (not too crazy) and follow the shape of his back.
Usually when you have less room/back space I thought a more open seat would be better, I guess it could depend on your body shape though. You have to be careful focusing on short enough for the horse because it can distribute the weight of the rider unequally/in such a way that is harmful to the horse. Mine had a saddle that was just a tad too long for about 2 years but never any pain or issue with performance. When I tried to shorten and go with a deeper seat, this caused more of an issue for us.