Here in the ADK's I don't use a traditional Kayak. For the St Lawrence, Lake Ontario and Lake Champlain, I use canoyak. A 13'8" version with a small kayak like seat. It has spray decks instead of a spray skirt, enclosing the bow and stern. I simply drop my pack in the back when I go paddling. Just use dry bags in the pack. I built it in my shop and it weighs about 19 pounds, making it very easy to transport. I use Yakima round racks. I have found the Thule square bars dig up the gunnels (the sharp edge) when loading or unloading it. I have used this across many of the streams, rivers and lakes of the ADK's as well as some larger waters, like the Great Lakes and Lake Champlain. The origonal boat was a 12' version and is better suited to smaller lakes and streams, but handled Lake Champlain on the Northern Forest Canoe Trail. Either version handles as easy as a kayak in the water since it is only 26" wide, and has a flared hull that turns most water, though, it can get a little "bouncy" on larger waves.
Anyway, a kayak is better for a lot of open water paddling. The little 13' boats do not track as neatly as an 16' boat. For most rivers and streams, though the manueverability of the smaller boat offsets the tracking. A straight line keel as opposed to rockered keel tracks better, but you loose the maneuverability. These two facter are at odds with eachother, manueverability and tracking.
Stability is another one. There are two forms: primary and secondary. Primary stability is how stable the boat feels as you get into it. Usually these are wider boats with flatter bottoms. These are usually poor performers since they "stick" to the water and can be dangerous in big waves. Secondary stability is how well the boat will handle wave action. The bottom of these are usually rounded, giving good bouancy in relation to wave action, but do not give a beginner good comfort, since they can be "tippy" to get into. Usually these are good performers. "V" hulled designs, "Oval" hulled designs, and others are compromises for good comfort and good performance. Most sit-on kayaks are quite flat. They do not have enough depth to the hull to provide rounding.
Raising your center of gravity will effect the stability of narrow boats. Sitting within an inch or two of the bottom will give you high stability. Sitting 6 to 8 inches up will tend to make the boat tippy. Sit-in kayaks are generally more stable than sit-on kayaks, though intial stability may seem higher with the sit-on due to hull design.
Solo boats will "bottom out" with performance at around 16-17'. Hull friction (with water) increases with length even though they cut through water more efficiently with added length. So, do not exceede these numbers for a good boat. Longer boats are used for better glides on touring boats, not higher speeds.