Quoted By:
Other posters explained the differences pretty well.. At the end of the day its all personal preference. Id say most really nice hiking packs are internal frames (For longer type stuff). They usally offer like more technology for supporting your back and what not. The nice external packs now adays and for more rugged users. Like I said though at the end of the day its up to you.
Heres an example of what I experience/have seen
>Internal Frame
Hiking AT, Thru hiking, 1Wk+ hikes
>External Frame
Hunting trip, Military, etc.
Pic related is the ruck I use. Now if i ever went on a hike longer than like 2weeks id get a really nice internal frame pack. They just feel nicer on your back nowadays. Now the pack I have, ive put through some shiiiiiit. Once I broke a frame and it still held for 20miles ruck running back.