I don't have an experience with ASSC but I do have a DownUnder that I lovvvve.
DU's little "we'll fit the saddle for you" thing is a little weird (they want you to place the tree over the horse's shoulders, which can hinder movement+cause pain, etc) so I'm not sure how much I would take that as gospel.
However! I have a DownUnder that my mare and I loveeee. It's a little narrow for her (it's a wide but she really needs like a extra-wide or something) but she does just fine. No soreness ever, and she's just all around very comfortable undersaddle. I also love it - super super comfy!!
I didn't have a huge budget when I initially got it so I went with the cheapest of cheap - the DownUnder Synthetic Endurance saddle. 2 years later, even though it only cost just over $200, it's still going strong. I used to ride in it probably 3-4 days a week in all kinds of weather - super cold, super wet, hot, you name it, it did it. I haven't had the time to ride in some time so I really haven't used it much more than maybe once in the last 6 months, but it still does it's job and looks nearly brand new - not beat up at all.
I've even ponied a really recalcitrant horse off it, using the d-rings in front. We even had to basically drag the ponied horse sometimes and the d-rings show no sign of wear!
I tried to avoid doing toooo much of that, but I think that's probably a good example of what it could do.
Anyway, I've been really really impressed with how well my DownUnder saddle has held up! I plan on buying form them next time i need a saddle.