We have a few using externals. For on-trail such as Philmont, they make a lot of sense. For one thing, they can be cooler on the back. I would highly recommend a duffel. Where are you planning on stashing all of your gear including that you are leaving at the trailhead? If nothing else, the stress that is being placed on the baggage is on the duffel and not the backpack you need for your trip. A tear or rip on the duffel is not the end of the world. For your backpack, it might be a different story.