"If you want a canoe that can serve as both a solo boat and a tandem, I'd get a 16 footer.  It is possible that shorter canoes will work well solo, but I've never yet seen a canoe shorter than 16 feet that will comfortably hold two adults and camping gear for a weekend. "

I disagree with that - I have done a lot of paddling in a 15' Grumman, many trips in Quetico of up to 10 days, with two average sized adults and all the gear - everything packed to fit below the gunnels; and hundreds? of day trips on local rivers and streams - works fine;  did the Allagash, up to and back down from A lake to the town - worked fine.  A 15' canoe is more nimble on rivers and small streams, though a 16 footer would be better on lakes.  Depends on weather you buy a canoe to fit all the gear you want to take, or you take the gear that fits the canoe.  For some people, they need an 18 footer to fit thier gear - whatever works for you.