The quick answer is: Both....
Each has its drawbacks and benefits. External frames can carry large heavy loads a bit better, and they are much much cooler when hiking in warm country. They tend to ride further away from your back and permit more ventilation in warm climates. People hiking in Montana rarely value this point, but in Az, it's worth something. Let's say you are going to circumnavigate Navajo Mountain up near the Utah border, climb up to War God Spring near the 10k summit, and then drop down to Rainbow Bridge where a boat filled with cold guacamole and mexican beer will be waiting for you. Daily highs will be about 85, and you will need to pack about 8 liters of water plus all your other gear. An external frame is probably the ticket.
Or, you are planning to hike down the Canyon in March on the Bouchet (sp?) trail which is about 30" wide thru the redwall, with sheer dropoffs on the right. The winds will be gusting to 30mph, and temperatures will climb to the mid 50s. There is water enroute, and at your destination. An external frame pack will act like a hang glider and you'll be worried that you may blow off the trail (DAMHIKT). Internal frame packs ride closer to the body, give you better balance off trail, and don't get hung up as much on tree limbs, and rocks. Climbing down Siphon Draw from the Flatiron in the dark with an external frame pack would not be fun, and the frame would end up somewhat worse for wear.
Internals may not carry extreme loads as well, and are more fussy in terms of packing and weight balance. You can load an external frame with a shovel and get away with it. Hiking thru slot canyons or off trail in heavy forests makes one admire how elk can run through trees with those big antlers...Internals are also quieter, since they don't have the mechanical pin mounts which can squeek annoyingly with externals.
I have both, and pick the best pack for the trip. The recent Supai trek required my Dana Loadmaster since I became the extra pack mule for our crowd (come to think of it the mule got paid.....) and carried 78 pounds down.
You have a short torso (but it is a very nice short torso....don't get sensitive...
), and will need to be fitted carefully to assure that a pack properly fits you and doesn't leave the shoulder straps 3 inches high when the hip belt is fastened.
You should fit a pack loaded (the pack, not you....) so take lots of full water bottles, a sleeping bag and a sack full of heavy gear so you can really see how it feels with a full load rather than empty (they all feel great empty). Dana Design (the Rolls Royce of externals...) and Kelty make very good external frame packs. For internals, the high end lines include Dana Design, Osprey, Arc'Teryx, Gregory, and North Face. I'm not sure who is making the internal frame REI models, I think Camp Trails or Kelty makes their externals. The Kelty internals are ok too. I think some of the current REi models are butt ugly aesthetically, but not everyone gets hung up on color coordinated ensembles like I do.
Its hard to pick one model that covers all situations from overnight boulder scrambling to thru hikes on the PCT.
Now, if they could only figure out how to get them to follow you on a leash, :roll: -Randy