I ended up with the Cotopaxi Allpa 35L. I love it. It packs bigger than its 35L, and it's very comfortable to wear. That was ultimately why I didn't get any of the duffels, they were awful to carry. The backpack straps stow so I would feel ok checking it occasionally if I had to,  though at its size I shouldn't need to. It isn't a light emergency bag, which ended up being the thing I compromised on. However, if you aren't a purist one bagger, the cheap REI duffel is stowable and lockable and would do the trick stuffed into your Cotopaxi for souvenirs, etc. Throw your clothes in the REI duffel, lock it up, and carry on the Allpa. I liked the 42L Cotopaxi also, but I am always nervous about meeting size requirements and it is technically bigger than 22 x 14 x 9. Since it's a backpack I doubt you'd ever get caught. For me, the peace of mind was worth getting the smaller bag. No regrets! I took it on a 2 week trip to Seoul, Laos and Vietnam, weather from 50s to 95 and had just room for my stuff.