Here are my thoughts are construction materials:
Plastic, ie royalex is great for bouncing down rocky rivers. They are relatively reasonbly priced, but would be nice if they were lighter.
Kevlar is feather lite, great if you'll be on a system of lakes and spending as much time portaging as paddling. They also hold their shape better, so should be a bit more efficient. Downside is cost, and not as durable if you really abuse the boat.
