Every major maker of packs uses good quality materials and stitches them together with all the requisite back-stitching and bar-tacks so that a pack is very unlikely to fall apart during years of normal use. Cordura nylon, nylon packcloth, ballistics nylon, and proprietary materials (generally similar to Cordura) are all plenty strong and abrasion resistant. Of more concern is the design of the pack bag— and this is largely a matter of personal preference.
Most packs designed for more than a weekend on the trail are top-loaders. The advantage: You can overstuff a pack up top by using a built-in extender, which gets capped off with a floating top pocket. And most packs offer a bottom sleeping bag compartment, so you can access your snooze sack without hauling everything else out in the process. But if you really want convenient access to your belongings, get a pack with a zipper on the main panel— either a side zipper, or for the best access, a full horseshoe.
Don't be seduced into thinking a flotilla of external pockets make for a better pack than a clean, nearly pocketless design. A multitude of compartments may help you stay organized, but they add weight and complexity. They also tempt you to add weight around the perimeter of your pack, instead of inside and close to the back where it is least likely to impede movement. I prefer a simple, single front pocket, big enough to hold a rain jacket, or a hinged 'shovel pocket' for carrying wet or bulky items. Add a couple of water bottle holsters and/or a sleeve inside to hold a hydration bladder, and you have all the extraneous compartments you really need.
Up top, it's handy if the aforementioned floating top pocket removes and converts to a fanny pack for day hiking or a summit push. Most packs have this feature. Fine touches to look for if you think you might need them: Ice ax loops, crampon patches, and lash points so you can strap on weird bulky things like snowshoes.
As for capacity, the 5,000-cubic-inch neighborhood should suffice for most of us— that's generally enough for a long weekend to a week-long excursion— and you can typically pile on another thousand cubes by way of the top extender. Figure on spending $200 to $420 for a quality internal-frame pack.
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