There are several key differences in the properties and performance between down filled and synthetic filled sleeping bags. Fortunately, these differences make it easy to decide between the two. In a nutshell the differences can be summarized by saying that a well-constructed down-filled bag would be smaller and lighter than a similarly rated (comfort rating) synthetic counterpart. Synthetic bags retain insulation qualities when wet (down bags do not) and dry out much faster. Finally, down bags are typically more expensive, though it pays to remember that down bags usually last longer. The price difference is due partly to construction methods, the higher cost of down filling and in many cases, the inner and outer fabrics used, which should be down-proof (to prevent down plumes bleeding through pores in the fabric).

Therefore, if size and weight is a critical issue, down filled bags are generally superior. If you're looking for a compact, light and warm bag, the chances are high that down is the way to go. Some people are concerned that their bag may get wet, and thus lean towards a synthetic bag. Usually these sorts of concerns should only be an issue if your tent leaks and or you do not incorporate some means of ensuring the gear inside your pack will stay dry (pack cover or pack liner). When we hear these sorts of concerns (which are often unwarranted), we often recommend that people consider taking measures to ensure that their tent won't leak and that anything stored in their pack will stay dry, no matter what.