A couple more items on down vs. synthetic: Down, properly stored and cared for, lasts far longer. Synthetic fill loses a little of its loft each time you compress it down to get it into your pack, so it will gradually lose its warmth. A high quality down bag properly cared for will last 15 or more years. The life of a synthetic bag is more like 4-7 years, per Mark Verber's excellent website.

Back in the days when I used an external frame, I always had problems--it seemed to have a life of its own and would zig when I zagged. I took a few falls because of this. An internal frame fits lots closer to your body and moves with you instead of against you--it's a lot harder to get off balance. Just be sure to get a pack that will carry the weight you want to put into it (found on manufacturers' web sites) and that fits you comfortably. Pack fit is almost as individual as shoe fit.

Have you read the articles on the home page of this site that we suggested above? They will answer a lot of your questions!