I have to agree with Solaris, though I don't have any experience on that saddle. I think that "soft" saddles are not good for the horse in general, regardless of brand. The saddle tree is made to protect the horse's back from the rider, and with no tree that protection is gone. A well-fitting saddle is better for the horse AND the rider, and keeps the rider where they should be. To help your aged mare, have a saddle fitter look at her, tell the fitter what kind of budget you have, and follow the fitter's advice. If the mare's back muscles are weak and prone to soreness, the last thing you want to do is flop around on them in a soft "anything". She needs the suppport of a WELL-fitting saddle.
And yes, an English saddle is lighter and shorter than a Western saddle, easier on the back. Even if you don't ride English, do it for her.