A:While the AT is an admirable goal, to be sure, there are other trail options. You seem to have seen a fair shot of the country, and thus know the relative merits of Western scenery versus Eastern scenery. Why not try the Pacific Crest Trail? Its more scenic, in my humble opinion, and is certainly less-traveled. But the AT has its appeal, as its a pretty sociable place.
Anyway, the same packs would suffice for either: large, but not enormous. If the budget allows, take a look at the Arc'teryx Bora 95 ($435), a classic big pack (5,500 cubic inches capacity) with an excellent suspension that successfully manages big loads (when food and fuel are the real issue). Access your gear through top or side zips, put wet stuff in an external self-draining pocket, and keep stuff organized with a sleeping bag compartment and other places for equipment. Its a nice pack.
Gregorys Whitney 95 costs $355 and has similar capacity. Its very similar in design and material to the Arcteryx, so you might find minimal practical difference in fit and comfort if you compare the two side by side. In this price range, take a look as well at the Osprey Argon ($370), which has a hip belt that can be heat-molded in the store for a just-for-you fit.
On the bargain side of things, Lowe Alpines TFX Appalachian 75+20 ($210) is a lot of pack for the money. Its rated at 4,600 cubic inches, but it expands easily with a tall collar for over-stuffed loads. It has a decent suspension for loads of around 50 pounds, and plenty of pocket and organization aids. Or take a look at REIs Mars ($199), which is a little larger than the TFX, has an easily adjustable suspension, and has nice touches such as a little pocket in the hip belt for stashing lip balm or other small items.
Try to get into a store and out on as many day hikes as you can, with a simulated load of at least 30 pounds. You want the hip belt to be snug, the shoulder straps to curve over your shoulders with no gaps, and no obstruction to your arms or head.
And the light shall inherit the Earth: If you're more of the fast-packing type and want to carry a lighter load, check out Away.com's how-to on the lightweight backpacking revolution.