For general travel, an internal.They keep your stuff together (though I still wish they had lots of pockets like externals for organization).
I used to use an external to thumb. It was awkward.I got one of the "horns " (the end of the frame at the bottom) stuck in a car door once.Keeping the sleeping bag lashed to the bottom can be tricky getting in or out of vehicles.On buses or planes I fret about the sleeping bag getting lost. I think Shoestring pointed out that externals won't fit in certain grainers.
If I was going on a really long hike,I would still want an external.Lots easier to pack, rides higher,You can always lash more to the frame.Also, in the woods, you can pull out the cotter pins that hold the pack to the frame,maybe use something to pad the frame a bit,and lash cargo that wouldn't fit in the pack directly to the frame.Good for collecting fire wood, hauling sacks of potatoes into Rainbow,etc.