We went through a canoe stage, and bought a fibreglass canoe which was about 15' long. We bought a long canoe because we thought it could carry the two adults and two kids in our family, that was our first mistake. Too long, equals too hard to handle, too hard to store, too wind effected on the water. The second mistake was buying a fibreglass one, we should have hung out and bought one of the newer plastic type ones, which are a little bit more forgiving with what you can do with them.
The other thing to make sure of is the type of seats which are mounlded into the canoe, try them out and make sure they are comfortable.
So my advice in short would be, don't go too long, get a plastic type, and make sure it has comfortable seats. And one extra would be a water proof compartment for your valueables when your on the water.
Hope our experience helps narrow the field down for you.
Our too long canoe has since been moved on. Have you considered the wide ski type things. Perhaps buy two.