If you had two bags side by side with the same amount of insulation (measuring by weight) you would find that the down bag is warmer than the synthetic bag. However, most people use synthetic because down is extremely susceptable to moisture. Even sweat and/or humidity can easily ruin a down bag. I have found that in general, if I add 10 degrees to whatever the manufacturer says that a bag is capable of then I remain quite comfortable in it. Of course I am far from an expert. I'm just a guy that has owned and used a lot of different bags. <img src="/images/graemlins/smile.gif" alt="" /> If it is just going to sit in a vehicle then I can't see the point in getting a down bag. IMO down is only worth the hassel if you need to stay extremly lighweight (where litterally every ounce counts) and you need to most warmth per ounce of weight carried. I am actually a bit of a lightweight backpacking nut (total pack weight often under 10LBS) and I still dont use a down bag simply because I cant justify spending the extra money on something so fragile and easily ruined, just to save a couple ounces of weight. My current bag of choice is this Slumberjack Superguide +30 . It's a lightweight bag that stuffs extremely small (I stuff mine to half the size mentioned by the mfr as the "official pack size") and under $100. Granted I live and camp in an area where it rarely gets below freezing for more than a few days per year so a 30 degree bag works well for me all year round.