Down is usually goose (better [smaller finer feathers] but more expensive) or duck down. Down is actually a cluster of thousands and thousands of interlocking, overlapping filaments that look like a dandelion. The reason why down makes such a good insulator is because of the loft (as mentioned above), which is the space between the filaments that traps and holds the warm air. Keep in mind that you might not have to think twice about making a decision if you're allergic to either type of down. Synthetic maybe the way to go then.
The quality of the down depends on the type, age, and maturity of the goose or duck. Generally speaking, goose down is considered to be better than duck; however, a mature duck's down is better than a young goose's down. As the age of the down is impossible for you to know, you have to depend on manufacturer's reputation and fill rating. The higher the number the better the down - for example an 850 down is better than a 650 down.
Down sleeping bags are very warm and comfortable because they are light and compressible. Down is thermostatic, which means it keeps you warm in the cold and cool in the heat (like most natural fibres, cotton, wool, etc.). On the down side (no pun intended) the bags have a habit of losing their insulating properties when they become wet. In fact, they can sometimes lose up to 80 per cent of their insulating powers when dampened. This is why it's important that you keep a down sleeping bag as dry as possible, and it's also the reason you should look for a model that has a waterproof, protective shell material. If your down sleeping bag does become wet, it will also take quite a while to dry out properly.
Another drawback to a down sleeping bag could also be the cost as they generally cost more than synthetic sleeping bags.
A down bag should only be washed with specific soaps and detergents, as the wrong kind could break down the insulating properties.
Synthetic sleeping bags are insulated with all types of "synthetic" materials, most of which you've probably never heard of. In fact, that can often be one of the drawbacks for consumers. The different types of insulating material can often lead to confusion because consumers aren't really sure what the fabrics are made of, and what their insulating properties are.
Synthetic bags are basically the only choice for those who are allergic to goose down or duck down. These sleeping bags generally insulate well, but they don't compress as well as down bags. In general, they also weigh more than down models and don't quite live as long, but they usually cost less.
On the bright side, a synthetic sleeping bag will usually still keep you warm, even when it becomes wet, and it will also dry out pretty quickly so you can use it again the same night. Most synthetic materials can be made very water-resistant. It's also easy to machine wash and dry a synthetic sleeping bag.
So, both are about as warm as each other, allergy considerations, wet/damp climate versus dry, weight, storage volume, care and cost should all be considered. However neither choice will be a bad one, as these two forms of fill both have strengths and weaknesses.
Having said all this, sorry to be a bit repetitive but this sorta summarizes it; I have had my down (Western) for over 40 years in all kinds of regimes from deserts, to the Arctic and the jungles in between. it has served me well and still use it always. Yes it was expensive even in those days.
Another advantage for synthetic bags is they are washable. But then so is my down, I use natural organic cleaner (woolite) on a lite cycle with tepid water and a couple of tennis balls in the dryer (low heat) to fluff (loft) the bag. Most people I know, that have down bags, take them to a dry cleaner which is about $50 plus.:costumed-smiley-007