I have both. I hunt with the EXO k2 and I use the HPG decker for training and general hiking. The frame height seems a touch taller on the EXO, which helps with heavier loads. Generally I like both, the way the EXO is setup is a little better for hunting. Decker is a little heavy. I don't know that it's much difference in durability. The Decker doesn't hardly have a lumbar pad, but it grips well. I have a flat back so it doesn't bother me much. Decker is also easy to remove the stays and fit them to you. If you have something giant to put on your back the Decker is going to fit it easier. If running the Decker pocket you will be using it to compress whatever you are carrying. I like the crib panel on the EXO so I can use that to compress heavy loads to stabilize them without worrying about anything in my pack getting crushed, or shifting and changing how the load is positioned.
I wouldn't sell your EXO before getting the HPG. Packs fit everyone different.
Short answer I don't feel much difference below 50-60#. Over 60 I think the taller frame helps me. For me anything much over 80# sucks no matter what, however it sucks less with some packs over others.
I'll check as I'm not certain that the frames actually measure any different, might be the way the individual packs ride on me.
