Colin Fletcher writes in "Complete Walker IV" that in his opinion, there's a qualitative difference between synthetic and down insulations of the same loft. I would say the difference might even be quantitative, with down being warmer than synthetic insulation per unit thickness. Some have said that the amount of loft is the only thing that determines insulating value, but is this really true?
If the insulation's purpose is to restrict the movement of air, then it seems some types of insulation might be better than others, for the same thickness. Synthetic fibers are small and cylindrical, while down fibers line up in sheets that are more 2D. In theory, you'd think a bunch of thin cylinders would be just about the worst shape for restricting airflow, while thin sheets might be more effective.
What do you think?