what are the differences between the two packs? Is the smaller one significantly lighter? Will you use the pack for winter camping or ever carry someone elses gear?

Don't buy a smaller pack just because its smaller. You need the one that fits the best and can carry your gear. A good pack has straps to compress it. My pack is 6500 cubic inches.
Jim cool
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These are my own opinions based on wisdom earned through many wrong decisions. Your mileage may vary.