None of the discussions of canoes and kayaks addressed the effect of weight and body placement on balance and handling. I had to learn some lessons the hard way:
Paddling solo requires sitting or kneeling near the center of the craft. You really can't load up a lot of gear in the front and paddle from the back, or vice versa.
Sitting and paddling only works well with a suitably constructed seat, with foot blocks you can really push against. Typical "standard equipment" canoe seats put your weight up too high, whereas factory kayak seats are much lower, and have foot blocks. So, for all-day solo paddling, kayaks have an edge ... if you are flexible and fit enough to sit in that position. You can have a canoe fitted with lower seats, but then you need the gunwales to slope inward so that you can get your paddle where it needs to go from that seating position.
Otherwise, canoe paddling is usually best done kneeling, with your rump against the seat or thwart. (To paddle a two-seater solo, face the other way and paddle it "backwards".) Invest in some kneeling blocks glued to the bottom of the boat.
The breadth of the bottom of the craft and the shape of the bottom are what determine stability (assuming center of gravity otherwise being equal).
Keels are not really needed unless your craft is built from wood or aluminum, which needs a joint there anyway. A nice v-bottom boat will track just as well. For an interesting compromise, try a shallow v-bottom, which by leaning to one side becomes a flat-bottom boat that turns easier.
New Jersey has some flat-but-twisty rivers that don't have waves, but require lots of turning. Paddling solo is much better for that because the weight is concentrated amidships rather than out at the two ends, making the angular momentum (if you know what that is) much lower.
The length of craft you need definitely depends on how heavy you are. The official ratings are based on maximum safe capacity, not on best handling. Ask your dealer to figure out which size is best for you, based on your personal weight and the amount of gear you typically expect to carry.
Hope this helps