Rubber.

Vibram Streamtread as seen on Simms, and Patagonia's new version of the rubber sole both seam pretty solid. The rubber sole is very versatile. Not perfect for many applications, but pretty darn good for most. Good choice for a "one boot to rule them all" sorta guy.

I just received my third pair of Patagonia Riverwalkers, second with the rubber sole (Aquastealth?) My only complaint is that the tread is a little shallow, providing less than awesome traction in deeper snow and soft mud, and it wears out a bit sooner than I would like. Granted, although my use should be measured more in days on the water rather than duration of ownership.

Good luck.