I have a 16" Clack with the tunnel hull and love it. It tracks real nice when your plugging and slides right of the rocks when you get into the shallows. I have never rowed a Willies, but I have rowed other aluminums and also a Hyde. Mine rows just as nice as the aluminums and better than the Hyde in my opinion.

There is plenty of room in the 16 footer and I don't see the point in going any bigger.

Zippy brings up some good points about mounting components to aluminum. It is a little more difficult in the fiberglass.

The Clacks are a little lower profile as well, so you may want to shy away from some bigger rapids although I have had mine through some decent size waves and it took them fine.

I doubt you could go wrong either way.