Buying a Sleeping Bag
One of the first consideration should be how you are going to using the sleeping bag. If the purpose is for car camping, then comfort should be looked at closely. However if you will be doing a lot of backpacking, you will want to consider size and weight.
The temperature rating of your bag will depend on the locations and seasons you plan to camp.Traditional bags are rated at 40 degrees F while three season bags should keep you comfortable at 20 degrees F. Cold weather bags are rated at 0 degrees F and lastly winter sleeping bags are rated anywhere from -15 to -30 degrees F. A ground pad or air mattress should be considered for comfort as well as extra insulation against the cold ground.
Besides for the traditional rectangular shape that provides more space for movement, there is an alternative. Mummy sleeping bags weigh less and are smaller in size. Along with a hood to pull around your head, mummy bags keep you warmer at night.
Another consideration is the insulation material. Down is a natural insulator and is lightweight. However it must be kept dry or will lose its ability to keep you warm. Synthetic fill sleeping bags are more inexpensive than down bags and insulate even when wet. They also dry quicker.