Internal or external frame? That is one of the biggest differences when choosing a pack. External frame packs generally carry your gear higher and are more adjustable for customizing your load. This makes them more suitable for heavy loads on the open trail. The frame on the outside keeps the pack off your back for better ventilation in hot weather, and provides a place to lash on all kinds of extras if you desire. The externals generally have more compartments for better organization, and on the average cost less.
Internal frame packs, on the other hand, are more streamlined for off-trail travel, especially in heavy cover where brush can catch on external frames. Also, the load rides closer to your body for more stability across rugged terrain. Internal frames are popular with skiers, mountaineers, winter travelers, and so forth. I prefer an internal for easier loading and unloading on canoe trips, and for the stability.