Welp, I rode her, got her sweaty, and am more confused than before.
She was moving out great, the same as she is in her Aussie (the Aussie fits her really well, could be a smidge wider but the world isn't perfect, she doesn't generally move out all that well in the other western saddle), the saddle didn't slide all that much (it did slide a bit going downhill but I was using a brand new wool pad and saddles ALWAYS slip going down those hills), and I really felt comfortable in it. She really seemed to feel comfortable as well and her stride seemed longer than it does in the other western saddle...
BUT, when we got home and I took the saddle and pad off, the sweat pattern was weird. I didn't think to bring my camera so no pictures, but there was sweat evenly on her back about 5 inches below her spine on both sides. Like there was the usual dry gap for the spine then a couple inches, then lots of sweat. No sweat on the tops of her shoulders/sides of withers or behind her shoulders.... The sweat was even, just more on her sides vs the top of her back...
The next time I ride her in that saddle, I'll try to take pictures... It was confusing.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.