I like both saddles very much. I like the balance (a tad more forward) and supportiveness of the XL H a little better. Those XL saddles saddles were of very nice quality (in their British bridle leather and their manufacture), and good, middle-of-the-road design. They had plain flaps and big, squishy, DelGrange-like knee pads. The XL H was supposed to be for the Hunter rider (as opposed to the one built with an even longer, straighter flap for the Equitation rider), but I think the flaps are relatively long on either model.
The fit for the horse will be somewhat different as there is more panel between the horse and the tree of the saddle on the XL saddles than on almost any Centennial. I don’t recall the wool-flocked Centennials (named the Internationale to denote wool panels?) being gusseted or that much thicker than the foam panels. I think the Centennial remains my all-time favorite saddle for the mutton-withered or sausage-round horse. But the panels on the XL will be broad and well-shaped. I think they are a more horse-friendly design than were the panels long built into, say, the Prix des Nations type saddles that came before the Centennial (and a few other minor redesigns that Walsall Riding Saddle Co./Crosby did).
I believe the panels in the XLs were a layered combination of wool and foam. But I could be wrong. The signs of wool-flocked panels are gussets (usually… those allow the panels to assume a 3-D shape for the shapeless wool to fill up) and slits cut into the saddle-side of the panel where one can insert more will. Feel around in there for those.
My test for a saddle’s geometry as it will help me has just two parts when I really get down to it: 1. Can I find a two-point and just hang out there all day? 2. Can I sit a big trot in it? I try to test-drive it at least twice.
If you don’t have the XL in front of you and you want a way to figure out if the flap will work well enough (at least to be worth shipping it to you), I’d compare a couple of measurements to my Centennial. 1. Length of the flap straight down from the stirrup bar, where the leather hangs. 2. From that line, horizontally out to the most-forward point of the flap.
Generally speaking, that will tell you how long and how forward the flap is. I suppose you could also measure the vertical distance from the stirrup bar to the intersection between the vertical and horizontal lines in order to let you now how high- or low that front point of the flap is. But I have a feeling that the XL H will have it’s front-most point pretty low. That’s similar in design to the Centennial, so you might just take the first two measurements and know the information you need.
Hope this helps.