The only advantage I can think of is easier to trailer and launch

I've sometimes wondered how much of a task it is to launch/rig/recover a 17. I've never timed myself, but suspect that it's around 30 minutes from starting to rig my 15 until I'm ready to sail away. The hardest part of that is heaving the engine around (which would be much easier if I remembered to use the 2HP Honda instead of the 5). I wonder, too, whether the 15 needs less water to launch -- I often find myself on pretty marginal ramps even with the trailer tongue extender. On the other hand, it must be really nice to have the extra room in the 17. I think of the M-15 as being backpacking on the water, but a whole lot more luxurious than kayak camping. Giles Morris