Synthetic fill outsells down probably 10-1, when I worked in a backpacking store in the 80s it was probably a lot more than that. Of course a large part of that is that given pros and cons on both sides the cost differential tends to be the real reason people choose synthetics. Synthetics do have a safety advantage, which is critical where bad weather exists, and the possibility of bailing to the roadside does not. While down is function most of the time, sooner or later something bad happens, and if you can't seek shelter it can be a life threatening event. But I think that is pretty unlikely while cycle touring in the NA. The worst weather I was out in occurred within reach of a motel.

What I like to use is a synthetic quilt. A lot of time during the summer I just use the bag as a blanket anyway, so using a quilt is a significant cost, weight , and bulk reducer.