I'm not familiar with your Mithril pack. However, if you're used to having load lifter tabs on the shoulder straps, you may want to also look at the Granite Gear Vapor Trail. I really loved the fit and comfort of that pack, and it handled well with loads in the 35 - 40 pound range (40 is pushing it, though.)
When I switched from my Vapor Trail to a frameless pack, I tried both the Granite Gear Virga and the Gossamer Gear Mariposa. (The Virga isn't a possibility for you - it won't handle a load over 25 pounds.) I ended up choosing the Virga because it had load lifters. The Mariposa's lack of such tabs left me feeling that the pack was always too far from my back and pulling away from my shoulders, and I kept tightening the shoulder straps - which moved the load to my shoulders and resulted in a much less comfortable ride than my old Vapor Trail. Talking to others who are very happy with their Mariposas, I don't think the pack was really pulling away. I think it was merely that I was used to the tight conformity allowed by the lifter tabs.
Other than that, I was very pleased with the Mariposa. It's a well-built, functional pack; I could just never get used to it not "snuggling" up to me, if that makes any sense.
You might also look at Ron Moak's Six Moon Design packs. I ordered one, and really was impressed with it. Never got to try it, though; he made only one size at that time, and it was too small, so I had to send it back.