There are two types of sleeping bags: down filled and synthetic material filled.
Down filled are naturally more expensive because of the high price of down. Manufacturers are developing better and better synthetic fillings which could match the features of down.
Sleeping Bag Guide The best feature of down is the excellent thermal insulation capability, on the other hand down sleeping bag loses its best qualities if it gets wet. The synthetic materials perform better in wet conditions. Down bags are lighter if you compare two sleeping bags with the same temperature rating.
The temperature ratings of sleeping bags are usually rated by the EN 13537 standard. The upper limit is the highest temperature at which an adult man can sleep comfortably without sweating and the comfort rating is the same for an adult woman. The lower limit is the lowest temperature at which an adult man is deemed to be able to have a comfortable night’s sleep. The extreme rating is a survival only rating for an adult woman.
Before you go to sleep in your sleeping bag it’s a good idea to move around and get the body temperature high. This way you get the temperature higher also in the sleeping bag. There are available separate sheets for sleeping bags which are nicer to your skin and also raise the temperature rating a bit. When you are buying the bag you should note that there are different sizes also in the sleeping bags. You shouldn’t of course buy too big or small bag.