I favour synthetic.The big disadvantage with down is if it gets wet it is absolutely bloody useless and takes forever to dry. That said, a wet synthetic bag is no more comfortable.
Overall, I tend to favour a down sleeping bag, what about you?
Yes, they're bigger, and bulkier but, if wet, they may not be comfortable but they still retain a large part of they insulating qualities.
A word for those considering down bags.
I am not a fan of down bags from an ethical point of view. I almost bought down from a company called Alpkit. I was reading their site and found that the vast majority of down bags are filled with feathers harvested through live-plucking. Birds are plucked then put back to re-grow their feathers to then have those plucked too.
Personally, I couldn't justify that just to save some pannier space.
Some Eider-down bags are filled from feathers collected from nesting sites but, given the labour involved, these are mega expensive.
Alpkit had said they were looking for a supplier of feathers collected from birds killed for food. When that happens, I may reconsider.
I do commend their openness and proactive attempts to make their products from more ethical.
This is my personal point of view on the topic, but it is up to others to decide what they do: at least now you have some facts...