I've used the GI large ALICE, it's not so much for camping/hiking as for muling a large load. Mules would probably not do the job, but soldiers think they are tough. The next step would be the SF type packs, which are really just older commercial designs. They have a reputation for not being as durable, which follows - few commercial packs could survive being repeatedly thrown off the back of a five ton for years.
Coleman made an excellent external frame, fully adjustable, called the Ramflex. It did well for the cost, offered a lot of versatility. I believe Outdoor Products picked it up when Coleman dumped it for another high profit market. That frame appears to be molded FRN, and offers a lot of durability and light weight. Packs that come on them from Outdoors are more modern, which may or may not be so good - lots of pockets sometimes are confusing, or worse, invite you to carry too much.
Internal frame packs were originally designed for rockclimbers - the kind of guys who sleep anchored to a rock face. The carry well, but don't offer as much adjustability. They are also more expensive, and part of the "I've got the same kind as those guys who climbed Mt. Certaindeath!" mentality. You see suburbanites unloading them from hybrids at the trailhead, packing wine and an icecream maker. They are not necessarily the best, just sell a lot right now. Maybe they have a higher profit potential as there is just a lot of sewing in them, not hardware.
Internal frame packs do sit closer to the spine - good for balance, bad in the summer when you need air circulation. The bendable sheets or sticks do bend into shape, and back out of shape, too.
Backpacking with any load is also a matter of getting into shape. If you plan on a longer trip, work up to it. A good reference point for all this is the keyword "Appalachian Trail." Dozens of books on the subject, with a chapter on proper gear in each. Probably the premier hiking event, and really tests the usefulness and durability of equipment - what little you actually need.
