Forgot to add. With either types of packs, the small details of weight distribution obtained by very precise packing make a lot of difference. More so in an external, as it is less forgiving. When I pack it right, I rarely feel any zig-zag. I have done 3rd class climbing in my Kelty. As well as winter mountaineering (on skiis). And it is all about fit-fit-fit! If the pack does not fit you properly, it will pull you over.