I have a GG Mariposa Plus - it works really well for loads 20-25 lbs and has an aluminum stay. Top load limit is 30 and I have pushed that - but it's only comfortable for me at 25. And I have to be very careful in balancing the load, whereas a pack like one of Granite Gear's Nimbus series that has a beefier frame, piling everything in any old way won't make me suffer overmuch.

I have a persistent shoulder pain these days so have been taking the Granite Gear a lot of the time. The weight transfer to the hips is better on a framed pack. Part of balancing the load with the Mariposa involves snugging up the shoulder straps - no load lifters - so some of the weight ends up on the shoulders.