Weight and volume are big issues during any outdoor travel experience. A down bag will weigh less and consume less space on your bike. They also cost more money. I have used my current down bag for over 30 years. I have worked as a canoe guide and mountaineering instructor during the summers in college. During all the years of use, I only got my bag wet once and that was on a bike tour. To dry my bag, I went to the town laundry mat, put my bag in a dryer with a tennis shoe and 15 minutes later had a dry bag. To keep the bag dry, put a trash bag inside your sleeping bag stuff sack, then stuff your sleeping bag into the trash bag/stuff sack. As far as sleeping in a mummy bag, the bag moves with you. If you bend your knees, the bag moves/bends at your knees, you don't bend your knees within the mummy bag. If you don't plan on using a good tent, then I would use a synthetic bag and use a compression stuff sack.