There are several different things to think about when you are deciding on which type of warm sleeping bag to buy. You've heard of synthetic sleeping bags, down sleeping bags, backpacking sleeping bags, but which sleeping bag is really the best. The better question would be: Which sleeping bag will keep me warm in the conditions I'll be in?
One of the first things you need to take into consideration when choosing a warm sleeping bag is your body size. The sleeping bag needs to be a little larger than your body, without too much space between your body and the sleeping bag. That space between the two is where the warm air is trapped, which keeps you warm. A mummy shaped sleeping bag is best for cold weather camping and a rectangular shaped sleeping bag is best for warmer weather camping.
It's also extremely important to know the average temperature and weather conditions for the area you'll be backpacking or camping in. A warm sleeping bag that's rated 5 or 10 degrees warmer than the average temperature where you'll be sleeping is best. If weight is a major consideration for your backpacking sleeping bag, use a lighter bag along with a sleeping pad to insulate the bottom of the bag. If it's winter or there's wet weather, a backpacking sleeping bag that has a water resistant outer shell is important. This shell should still be breathable though, so you don't get wet from your own sweat. Sometimes a bivy bag is used over a sleeping bag to stay dry or to make an extra warm sleeping bag.
Another consideration in choosing a warm sleeping bag is whether to buy a synthetic sleeping bag or a down filled sleeping bag. A Synthetic sleeping bag can be heavier and less compressible. However, they can also be less expensive, are easy to clean and they still insulate quite well when they're wet. Down sleeping bags on the other hand are lightweight, a very warm sleeping bag and breathable, but they are harder to clean, cost more money initially and aren't very insulating when wet.
In a nutshell, there are several things that need to be taken into consideration when buying a warm sleeping bag. First, choose a sleeping bag that is the right size and shape for your body. Next, do your research and find out the average temperature and weather conditions for where you're going. Finally, decide if a synthetic sleeping bag or a down sleeping bag will best suit your needs. If you've done those things, then you're ready to buy your ideal warm sleeping bag.