23 years in the Infantry taught me that internal frame packs are the most comfortable, but external frame packs are the most versatile.

I've used both to pack out elk quarters and by far prefer the external frame. It's easier to lash quarters to, you can remove your pack and take just the frame and a daypack along with you on day hunts. With an internal frame, you're basically stuck with carrying the complete pack, which is more bulk than makes sense to carry.