do you have some way to try them on you? at a camping store,mil.surplus store perhaps?
it kinda depends how tall you are and how long your back is,how does the bag feel when you have it on you? are the shoulder straps okay,are the starps wide enough,does this bag sit properly on my shoulders,zippers good,how's the stiching done,would think most mil.surplus stores have a big variety of bags and some are nearly new or very little used.
my BOB is and old model saddel rucksack,canvas with leather straps and still missing that frame ( now know where to get one).
but whatever you get,be sure material is good,stiching okay,it's a hell to carry something that's broken after a few miles,I'll try to dig some more info for you, perhaps I find some pdf here ( should be here some place)