I think it boils down to personal preference. In my case I went from a 50L pack to a 34L and found that the compressed dry sack left too much unused space in the pack. I was attaching the dry sack externally until someone here suggested to try and stuff it straight into the pack. Now I can fit everything inside which makes it alot nicer not having 3lbs dangling from my packs bottom. Prior to that I was carrying a square rucksack style pack which easily accommodated a stuff sack full of down.
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