You can comfortably strap quite a bit more onto the outside of an external frame pack than you can to an internal framepack. But you can still strap some amount of stuff to an internal frame pack too, so you'll need a larger volume capacity inside, but maybe not quite that much larger.



My experience is that the typical internal frame packs (Osprey, Gregory, etc.) are significantly heavier than their external frame counterparts. Their advantage is that they hold the weight of your load close to your body, which makes the whole bundle feel lighter on your back.

However, there are lightly framed or unframed packs that are much lighter than typical internal frame packs, such as the Vapor series from Granite Gear, GoLite packs, etc. These will typically save you quite a bit of weight over an external frame.