When I was choosing storage for a car kit, I found something at Wal Mart that I thought would be big enough. Then I ran around the auto department grabbing bottles and jugs of the fluids I wanted to carry in it. Got some strange looks btw.

Anyhow, when I packed it all in there I tried to decide if there would be enough room for the other things that were not close by in the auto department. Then I put back all the stuff and bought the container.

Point being, if you already have your gear assembled that you want to carry in the backpacks, you could (after confirming the return policy) order the bag, stuff it and evaluate the comfort of carrying it as well as the (a variation of my favorite word) robustness of the pack. This may help you predict possible problems with the pack in a real life situation too.

You mentioned Maxpedition, and although I don't have any of their packs, I think the modular design of their products would be a plus...adding additional storage as need or situation warrents.