RE: Sleeping Bags
IMHO Wiggys bags are by far the way to go. Walk in and get 40% off to boot. FRTSS and you get 2 bags in one.
As to down in a bivy bag-- the bivy bag won't help if your plane goes down in the lake or if the raft flips and its not in a waterproof container.
Those are 2 reasons I prefer Wiggys. They can save my life. Weight is not bad either. Even keeps my cold natured wife warm at nite. If I went with down it would be for the weight savings. Adding a bivy to that would mostly defeat that idea in weight. But it all depends on what you are doing and how you are doing it.
Of course its all personal opinion.