I'm going through this in my head - maybe it will be useful to someone else!
I have a hard time getting my thighs/knees on the horse in the Eurolight; instead, they're flared out. This is less obvious with saddlebags. This is also likely the reason my canter is worse in the Eurolight. Three Eurolight rides later: I realized that my knees are currently on the lower D-rings.. which is where I've had my saddlebags rigged to, which would make putting my knees on the horse impossible. I don't have a solution yet, but the bags are still off right now.
The Eurolight has a deeper seat, and generally makes me feel more secure. That said, it also has a steep cantle that I frequently feel myself running into. If I lean back in the Thorowgood, I'm leaning back. If I lean back in the Eurolight, I'm fighting the saddle. I suspect it's probably equivalent to a 17" English and a bit small for me.
I am tending to lean forwards in the Eurolight. After switching to non-caged stirrups that feel longer: issue (mostly) resolved.
I'm not yet feeling secure in my caged stirrups - I feel like I'm fighting those too. I may put my old stirrups back on to see if that resolves anything. I LOVE the idea of the caged stirrups.. but I feel like I cannot get my foot far enough in (or at least my left foot, right foot does okay). I adjusted the placement of the left stirrup leather and Saturday's ride should tell me if that fixes things. The fact that I can get one foot happily in makes me think it ought to be fixable.
After Saturday's ride: nope. Not quite fixed. Better, but not quite fixed.
Sunday's ride: switched back to non-caged stirrups, and I feel way better about life. Our canter was halfway decent. I can post comfortably. I'm not having to think about my position every three steps to figure out why my foot feels insecure. My canter seat is.. well, it's better with the non-caged stirrups than the caged, but I think it's still best in the Thorowgood right now.
I don't have a good answer for why the caged stirrups are not working for me. I suspect it's a combination of stirrup height (effectively changing my total stirrup + leather length) and trying to get an extra half-inch of foot through the stirrup. I have big feet (womens 9.5) and perhaps this is one of those things where it's actually going to matter. Or maybe it's also partly that I need to drop the stirrups a half-hole to account for the firmer stirrups, rather than trying to ride with hunter-type knees.
ATG would probably make me bigger cages if I asked. At this point, my spare cash is gone and I have a ride in four weeks, so for now, that will wait.
After two weeks of bareback/dressage saddle work, the Specialized with Woolback feels awfully wide. I'll try at least a short hack with an English pad to see if that fixes my issues with contact. It may cause bigger problems with pony-saddle-fit, but that will at least answer some questions.
After Sunday: Hahaha, no! No I will not. I am not capable of easily getting the saddle fitted with an English pad rather than the thicker Woolback. There is a dramatic difference in fit (bridging! major pressure on shoulders! something wrong towards the back!) and frankly, if I have to spend more than 30 minutes fussing with saddle fitting to get it to work this close to a 25, I may as well ride in the Thorowgood. But. The Specialized as-is fits the horse even if I'm not quite sure about me, so we will make do.
Incidentally, today I read Gail's post about her Specialized demo, and yep: nailed it. It's a wider twist. It never bothered me before, but I went from a crazy-wide western-endurance saddle to the Eurolight, so it was probably an improvement then. This was the most English riding I've done in two years.
Something about the buckle or twist of the right stirrup is still making my leg unhappy. I still don't know why. I may experiment with moving that buckle up six inches.
Buckle moved! I didn't notice it, so maybe that will fix things, or maybe I was too busy being thrilled with the stirrups to notice.
So, after reading all that again, the obvious question: why not just ride in the Thorowgood?
1. It fits. But it doesn't fit as well as the Eurolight. I notice it riding her shoulders more on the downhills, and laterally I don't feel like it's quite as stable. Do note, though, that I nearly ground-mounted with the Thorowgood sometime in the past few weeks, and it stayed just fine.. so that lateral fit may be all in my head.
2. I want a bigger pommel to run into when she does dumb things like screech to abrupt halts. I can sit everything in the Thorowgood if I have to, but that's all my seat and not the saddle. Sometimes I want help from the saddle. All her bucking and hissy fits last winter? I would have been eating dirt very early on in a dressage saddle.
3. The Eurolight is more forgiving when I'm tired and not riding as well. Again, I probably could ride in the Thorowgood, but it's going to take a toll on me and thus on her.
4. The Thorowgood makes my knees unhappy after a while. Probably fixed by switching to endurance stirrups, but see above about financial status: broke.
5. Eurolight is way more comfortable several hours in. Possibly somewhat fixed with a full-saddle sheepskin cover, but again: broke.
6. I don't have to canter right now. Would I like the option? Sure. Do I need it for my next ride? No, I do not, and we're well capable of trotting all of it instead if that's what it comes to.