The main differenve between down and synthetic fiber sleeping bags is that down is lighter than synthetic fibers and easier to compress, which means down sleeping bags fill up less space. They are also more breathable than synthetic sleeping bags and that makes them better at regulating temperature.
The advantages of down sleeping bags
Down sleeping bags are more durable - if they're washed and handled with care, they can last to up to 20 years or more. The durability depends, of course, on the quality and the usage frequency. A drawback of down sleeping bags is the relatively high price, compared to the alternative synthetic fiber sleeping bags. In addition, they are also not optimal in very humid environments, as the down can clump together. However, there are the Sea to Summit down sleeping bags, where the down is treated with a particularly water-repellent polymer, which repels moisture and causes the down to dry up to 60% faster than untreated down. This way you get the best from the down sleeping bag, naturally for a higher price.