There are several factors to consider when choosing a sleeping bag including shape,size, temperature rating, and type of insulation.

Shapes

The two basic shapes are mummy and rectangular. Each has its advantages and disadvantages.

For those who are somewhat claustrophobic, a rectangular bag may be the way to go. It allows sprawling out without confinement. The downside is that rectangular bags are usually heavier and require greater weight to get the same warmth as a mummy bag.

Mummy bags have a closer fit and a hood that seals over the head. Because they have less space inside, they insulate more efficiently (less air space for your body to heat) and weigh less. However, they can be somewhat confining.

Sizes

Sierra Trading Post lists the dimensions as well as the recommended maximum height for all of our sleeping bags. If you'll be doing a lot of winter camping, choosing a longer bag that allows some storage space in the foot might be a good idea. This lets you store things like stove, socks, and other essentials that you wish to keep warm over chilly winter nights.

Temperature Rating

You should choose a temperature rating based on the conditions you'll be camping in most often.

40°F tends to be used for warm weather camping and is the rating associated with most traditional bags.

20°F is considered a versatile three-season bag, usable in many climates from Spring to early Fall.

0°F is used for cold weather and some winter camping.

-15°F to -30°F are suitable for camping in extremely cold, winter conditions.

Insulation: Down vs. Synthetic

Sleeping bags are filled with either down (from geese or ducks) or with synthetic fibers. Each has its advantages and disadvantages.

There's really nothing better than down. It's light, compressible and maintains its loft. However, it won't insulate as well when wet (losing up to 80% of its insulating ability), and it takes a long time to dry. This drawback can be pretty major if you spend time camping in wet conditions. It also costs a little more than synthetic fill.

Synthetic fills, such as Polarguardr and Hollofilr also insulate well and are great for people allergic to down. They don't always compress as well, weigh as little, or last as long as down bags, though. However, they'll keep you warm when wet and dry quickly.