Generally, the decision between duffle and pack ought to be based on how you intend to carry it. Since the primary consideration is that I will eventually have to sling it or carry it by hand, I would think the pack to be more ergonomically convenient. I don't see where a duffle has any other particular advantage over a non-frame pack of equivalent capacity, so the backpack gets the nod there.
Beyond that, you really need to decide first what your BOB provisions are before determining size, compartmentality, etc. For what I need in my BOB, I can do with around 2,000 cubic inches of space in two or three compartments. That includes first aid supplies, a change of clothes (sans shoes), a couple knives, three flashlights, some firestarters, a bunch of other survival gear, a little food, and enough water for a couple days. That still leaves enough room for yet more stuff, but all that water makes it a little heavy, so I would pick and choose to fit my timeframe and situation beyond that short list. All this in a 2nd hand daypack my daughter no longer wanted to use for school.
Mind you, I have my edc with other stuff in it that augments my BOB quite nicely. I've traveled through airports with both slung, one over each shoulder. It's noticable, but I can carry them around all day if need be (I had to a couple times waiting for late flights already).
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)