The pros and cons are well known in this case:-
The weight and compressibility are the main practical factors, but we find the luxuriously soft warmth of down unbeatable. We now use Mountain Equipment down bags which have a water-resistant Drilite shell - useful for repelling condensation (and frost showering down inside the tent in winter!). All-year camping really needs at least two bags, basically a summer one and a winter one. The ideal would be three: winter, mid summer and the rest of the year. A cheaper alternative might be a medium-rated bag for most of the year and a fleece liner or similar to increase its rating for winter, but this is a poor option that also increases weight and bulk.
This is a real minefield. Bags are (or were) sometimes assigned a rating in seasons, though these days they are usually given temperature ratings. These can be very misleading because there are so many dependent personal factors:-
Then there are the practical variations, among others:-
Interpretations of ratings will differ between manufacturers and we don't take the figures seriously. There was one interesting result from someone who had the chance to test a whole range of bags - he noted that all the ratings were on the optimistic side, but those of synthetic bags were particularly optimistic.
We wanted to get the really expensive winter one right first time. We decided that since we might be pitching at high levels in mid winter when the valley temperature was -10C say, we wanted some insulation in reserve. Accordingly we bought the ME Iceline which was the second-highest rated bag from the range at the time (-25C). We were not disappointed, this bag is simply superb and toasty warm. For the lighter bags we chose the ME Lightline.
We see remarks on newsgroups and forums on this subject that are obviously posted by people who 'run hot', and they seem to think that everyone else does too. There was one which confidently asserted that a 1-season bag was adequate for most of the year. If that is true for them, then that's fine, but there are many people who will be in for a real shock if they make their first mountain camp in Spring or Autumn with a 1-season bag. We have received a couple of sad reports from readers who have been swayed by these and made their first trips with an inadequate bag, relating tales of cold sleepless nights and feeling really drained in the morning. We don't run hot and a 1-season bag means what it says, and even then there can be some pretty chilly nights in summer. Our ME Lightline bags cover a wide temperature range and we can use them with total confidence from around Late April/May to October.
There is a new ratings standard EN 13537 under development which aims to give better consistency in the temperature claims. There are 4 temperature levels quoted under the standard:-
All ratings assume that you are cocooned in the bag like a mummy with the zip fully closed and the cowl drawn over the head. In practice it's not so simple. We leave the zip partly open to get an arm out and we don't normally use the cowl because it doesn't move with the head when we turn. In cold conditions we use the detachable hoods of our down jackets and wear those - they prevent heat loss from the head and are very soft, warm and comfortable to sleep in.
Some modern bag designs save weight by reducing the space inside. They feel pretty tight around the body, which is good from the insulation point of view but allows hardly any room for movement, which makes it difficult to get a comfortable sleeping position. Some designs save further weight by not having a zip at all, but this reduces its versatility:- opening and closing a zip provides a very simple means of temperature adjustment.
The warmth to weight ratio for down bags is largely determined by the 'fill power' of the down, which gives a measure of its lofting ability for a given weight - the higher the fill power, the warmer but more expensive it is. The ME bags use 700 down which is very warm and high quality, but Marmot now have the Helium and Lithium bags that use an incredible 900 down, giving the highest warmth to weight ratio possible.
All quality bags are designed to keep the filling spread uniformly over the bag and prevent it collecting, which would give rise to cold spots. Reading the design claims and terminology about this, we can't distinguish the real benefits from the hype, but the EXL elasticated stitching used by the ME bags, which gently pulls the bag inwards to the body, does seem to feel a bit more cosy. One tip here about packing the bags, certainly for down and maybe synthetics too - don't fold them up neatly. They should be stuffed into their stuffsacks in a haphazard fashion, this helps prevent concentrations of the filling and keep it evenly distributed.
When choosing a bag for winter, a freezing high level pitch is not the place to be hit by the double whammy of feeling cold and realising that you made an expensive mistake. It is much easier to cool down a little than warm up - think carefully before shelling out!.
Another desperate attempt to save weight, these are bags with no down at all in the underside section: a mattress is slotted into the bag and all the filling is above it. The idea is that the down at the bottom of a normal bag is flattened and provides no insulation, so it can be removed and weight saved.
Like so many ill thought out ideas, this argument is far too simplistic in most situations. Flattened down does provide a little insulation and comfort, moreover your recumbent body is not like a flat rigid board that compresses the entire bottom section. It is a complex shape with pressure areas and hollows, the down will be compressed at contact points but it will loft and insulate more in the hollows. More importantly, as you move and turn in the night a normal bag moves with you: down that was previously compressed at the edges will loft and maintain the insulation as you move, which cannot happen with this type of bag.
The mattress keeps the interior of the bag at maximum width, which may prevent the down from snuggling close to the body and you may not be able to draw the top end in around your neck. Sleeping on your side will also produce an open void on either side of your body where cold air can get in and circulate.