I love the way the Stone Glacier frame has the meat shelf built in and you don’t have to depend on the bag for that. I also thing the Stone Glacier frames are the easiest to adjust with the velcro yoke and each shoulder strap being independent. I think the Kifaru frame and modularity allows for more versatility once you get the frame to fit you; their bag and accessory options allow you to do exactly what you need to do. Seek outside has a simplicity about it that makes it appealing, but I didn’t go that route.
I didn’t like the Mystery Ranch frame…it squeaked a lot under weight and wasn’t easy to customize the fit.
My advice is to focus most on getting the frame to fit well and test it under weight. That would require trying multiple frames and being willing to adjust and adjust. After that’s figured out, look at the bag options as all these companies have many routes to go.
I ended up going with a an SG 5900 on a xcurve frame. That’s my main backpacking bag. I added a sherman pocket in blaze from Kifaru and it performs well, also added a bungee cord cargo to allow items to dry and keep my camp shoes out of my bag. I kept my Kifaru frame and have been whitetail hunting and using it for training with the Straight Jacket, which has been awesome.