Down is the original fill for insulated jackets and sleeping bags, and is in many ways still the super hero of insulation. It comes from the under-coat of ducks and geese, and is in the form of light-as-air clusters, not feathers. Down comes in a range of quality (find out more about Down Fill Power), but here are some general traits: Pound for pound, nothing insulates like good down.
Insulation is created by trapped air space, and down lofts up to take up more space than any synthetic out there. That means that a down jacket will be lighter weight than a comparably warm synthetic. It will also be easier to pack and will stuff down smaller. Down also has the advantage of durability.
Properly cared for down (learn more about How to Care for Down) gear can handle being stuffed and unstuffed hundreds of times, and can last a lifetime. Last but not least, down is comfortable; it is hard to beat the feeling of being enveloped in the light, soft warmth of down, and down's higher breathability gives down gear a little broader comfort range than synthetics.
So what is the weakness of this super hero? Moisture. Drop your pack in the drink, wet the bed, or be dumb enough to leave your bag out in the rain, then your down gets wet and you freeze. Wet down loses its loft and all of its ability to keep you warm. It also takes a long, long time to dry. There are many ways to protect against this eventuality, from smart packing to various fabric technologies. But the risk is there, and that leads many people to look into synthetic alternatives.
The other principal drawback to down is that you may not be rich: Down is more expensive than synthetic insulations, particularly if you are looking at the higher quality, higher performance fill power ratings.
There are a few drawbacks to synthetics. As has been said, they are not as light or packable as down. They also tend to be stiffer in feel and so not as comfortable in both clothing and sleeping bags. The other drawback is longevity. Repeated stuffing and unstuffing of synthetic fibers has the tendency to damage them and cause them to clump up, undermining even dispersion of insulation and causing cold spots.
The answer is... it depends. Take a look at the pros and cons, think about what you will be doing, and make an informed choice.
If you can afford it, down gear will end up the favorite for most applications. Down is great for drier areas, or places that are too cold to be wet. It is a solid choice for people who are going to put a lot of use on their gear and want something that lasts. It is also the go-to option for the ultra light crowd. Another consideration is activity level. If you are looking for something to keep in your pack and pull out to keep you warm while you are stopped to rest, eat, belay, or whatever, down is an excellent choice.
But, if you are looking for a jacket to wear for cold weather skiing, where you alternate between sweating on the slopes and freezing on the lifts, synthetics make sense because they will better manage the moisture of you sweating in them all day. Synthetics are also the right choice for people who spend a lot of time in wet, wintery mix weather. They are a good idea for kids, as the ability to regularly wash them is important. People on a budget, people with down sensitivities, and vegans will all find themselves happy in the market for synthetic fill gear.
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