I dont know much about the San Juan, as I am just getting started on WW, but I have been stand up paddling for about 6 years now.
If the river isnt boney, you'll be fine with a composite board, so long as no one minds bumping into 10' of board in the raft. There is no way I could run the Poudre in anything other than an inflatable, so I have 2. It's nice to be able to stow them for long periods of time, but the performance sucks.
Come check out the event over Memorial Day at the Glenwood wave. Try them out, see what you think is going to work for you, talk to people and see what they think.