For me, I bought an external pack, a Kelty Trekker 3900. I will also be doing the AT in sections, but it will be in June and July. Internal frame packs hug your body, and thus create more sweat. An external frame leaves some room between your back and the frame, allowing airflow, and less sweat. Other than this difference, the pros and cons really cancel each other out. If you do most of your hiking in the summer, I'd suggest an external pack. A two time thru-hiker gave me that advice. I'm really not sure the frame on a pack makes that much of a difference. Just get whatever feels best for you.

The Kelty 50th anniversary is 7 some pounds, but if you take off the outer pockets, the weight drops to around 4 pounds.

hope this helps