All out packs have torso adjustment, so either will work fine. The BC will have more capacity and a bit more weight carrying ability due to the dual stays. Otherwise it comes down to design.
Thank you for the quick response. I've definitely narrowed it down to these two models. I like the Aston House a bit better due to the water area (I use a bladder) being seperate, but I can see the Umlindi being good for shorter hikes where you just put your gear inside and go. I'll likely end up getting both and use the Umlindi for shorter hikes and esp for travel, since the Aston Pocket fits so well on the Umlindi as well.
Thanks for making these really well thought out packs.