Once again, you need to think about its use, the time of year you will be using it, it's weight, what it's made from, size and shape. I bet you never thought there would be so much to think about when looking at buying gear. But the reality is, just like boots and packs, there are factors you need to consider when buying a sleeping bag.
Let's look first at your size and shape. You won't need a really long bag if your short. You can get wide bags if you're of big build. But for the average sized person shape isn't too much of an issue. However some people like square ended bags while others don't mind tighter fitting 'Mummy' type bags that are tighter around your feet. You can also get bags that can zip around the bottom of the bag to allow your feet to stick out or allow air follow. This is good if you get hot feet.
Three different sleeping bag styles, L-R 1-'Mummy' 2- 'Rectangle' 3- 'Square'
Look at the time of year you will be using the bag. For example you won't need a really good winter weight bag for use in summer conditions, it will just be too hot. So if you only plan to be doing your adventures over, say late spring, summer and early autumn then a summer sleeping bag is all you are going to need. These bags are light weight and generally used to just keep you warm/comfortable at night.
My winter bag L, summer bag middle with silk liner L
If you are doing trips where weather conditions start to get abit cooler you can carry a lightweight silk liner to add that little bit of extra warmth and you can put on say some long-johns and socks to add warmth. A silk liner or Thermal liner is a good investment to add warmth but also help keep your sleeping bag clean from sweat, body oils/fats and odour. It's much easier washing a liner than your sleeping bag.
My Silk liner, it might add a few extra degrees of warmth and weight very little.
A winter sleeping bag is another purchase, as if you are going to head out in winter conditions having an appropriate bag specifically designed for these cold conditions is a must. Winter bags are often around that 1.6-1.8kgs so you are carrying alot more weight in your pack but the plus side is your carrying alot more warmth. A good quality winter sleeping bag could set you back anywhere from $600-1000. But in those very cold conditions and if you're going to use it alot, money well worth spent. In really cold conditions having a good winter bag is essential.
Look at the style of bag and make sure it has a hood that can be pulled around your head to trap in the warmth.
My winter bag, nearly 2kgs but in cold conditions worth carrying.
Some bags will only have filling on the top half of the bag. Don't buy one as really you need warmth on all sides of the bag. These are made to reduce weight however they are not going to keep you as warm.
Really there are two main types of sleeping bags 'fill' as in what they are filled with to keep you warm. 'down' or 'synthetic' Down is usually form goose or sometimes duck where as synthetic is manmade. Are there any differences between the two? well that's hard to say really the only way would be buy one each and test them out to see which one you liked best.
Personally I use 'down' bags as in my opinion find them warmer. But I'm sure a modern synthetic bag would be just as good.
One down side of 'down' bags is once wet they are pretty much useless as the down just turns into a soggy mess. Whereas synthetic's will hold its shape and can still have a level of warmth. So with that in mind you should always put your sleeping bag into a 'dry-bag' inside your pack, so that if your pack got wet you know your gear inside your pack will remain dry and safe inside that dry bag.
I have two bags. A winter bag and a summer bag. I also use a silk liner in my bags for that added warmth and to protect the bags. My summer bag weighs 760 grams while my winter bag weighs in at 1870grams, over twice the weight of my summer bag. Once again ask what friends use, shop around and don't buy a cheap bag.
L-R my winter bag, summer bag and and older mid range bag.
Invest your money in a good quality bag, it will last a long time and probably have all round better everything as opposed to a cheap Chinese knock off or one from the Warehouse.
Big stuff sacks used for storage of the sleeping bags.
And like everything, look after it, keep it clean and when home from those adventures, hang the bag up don't leave it still squashed up in it's carry bag as this this reduce it's life and warmth. The better you look after your gear the longer it will last.