Features to consider:

• Size of storage compartments.
• Front-loading vs. top-loading access.
• Comfort of shoulder straps and waist belt.
• Conformity with airline carry-on size limit.
• Durability of fabric, fasterners, and zippers.
• Weight.
• Detachable day-pack.

Sarah decided on the Lowe Appalachian Trekker. It has 4200 cubic inches of storage separated into 2 main compartments. I went with the "North Face Galileo" model -- 3600 cubic inches plus another 1000 in a detachable day-backpack. The North Face features an integrated cable that can be used for locking, but unfortunately it results in a much heavier pack. The Galileo also has a built-in raincover.

The most important factor in both our decisions was fit, especially in the torso length. If you're in the market for a pack be sure to try them on with weight in the pack. We both changed our minds after trying on the packs we thought we would like and realized that they didn't fit correctly. The next-most important factor was to find a front-loading pack. Front-loading packs allow you to unzip the entire front of the pack, rather than making you dig down from the top.

Where To Buy?
REI has a large selection in their travel section. They feature REI brand, Lowe Alpine, Eagle Creek, and Jansport, but they don't have many North Face models. If you are visiting an REI store, be sure to go to their TRAVEL PACK section, not their main BACKPACK section.

Other retail stores to try -- The North Face (high quality and price), luggage stores (lots of wheeled varieties), and camping stores (primarily top-loading backpacks).