Hey All, this may be a long winded post, however...
I would agree with some of the posters here, I will add types of bags and the pros and cons of both synthetic insulation and down, and a few other notes:
I have owned several types of both insulation bags, but have pretty much upgraded all mine to down bags (have about 10 ranging from 4c to -40. Lol
You either can get a mummy bag, barrel bag (sacrifice thermal efficiency for more room/comfort), or rectangular bag (same as barrel)
Mummy
Mummy bags are designed to save weight and maximize heat retention. The tapered, narrow shape means that your body has less space to heat inside the bag. Most have an integrated hood that you can cinch closed over your head to keep heat inside. They offer superior warmth to weight than a roomier bag. However, some people find them too constricting to get a good night’s sleep.
•Lightest weight
•Smallest pack size
•Most thermally efficient shape
Barrel
A more relaxed shape than a mummy bag, they are more spacious and often don’t have a hood attached. A good compromise if you like to sprawl a bit when you sleep, but still want a light, compact bag.
•Heavier
•Larger packing
•Less thermally efficient than a mummy bag
Rectangular
Great for warm weather, a rectangular shape is the roomiest option but the shape creates additional space inside that bag that your body has to heat. They are best for car camping and for sleeping out when the temperature stays above zero.
•Least thermally efficient
•Roomiest
•Can be opened as a comforter
•Bulky and heavy
•Easy to zip two bags together
Insulation
Pros of Synthetic:
Cheaper $ wise
good in damp/wet weather
hypoallergenic
easier to wash/maintain
Cons of synthetic:
Heavier (than down)
Bulkier (for same temp rating)
Do not last as long as down (tend to get "cold spots" where insulation has shifted/broken)
Pros of Down:
lighter (for temp rating)
less bulky (by far)
last longer
Cons of down:
Pricey $$$ (usually 3x or more)
not as good in damp/wet weather as synthetic
not as easy to wash/maintain
For down, fill-power is a good indicator of quality. It’s expressed as the number of cubic inches an ounce of down displaces. Higher fill-power provides more warmth for the same weight.
•500-550 is OK
•600-700 is good
•750-850 is excellent
As for my comments earlier about upgrading to down or buying down for that matter. It is all about the affordability. Down bags to last years longer than synthetic, so there comes a point of depreciation where it will be close to cost, however this is relative to how often you use it. Personally I always would recommend saving up and getting a good quality down bag. Nothing worse that having a craptacular sleep because you wake up cold, and uncomfortable and realize you still have days to camp.
For sleeping systems I would definitely recommend a sleeping pad (I have a exped downmat 9) like others, but they are definitely NOT cheap. For altitude, you may want a closed cell mat (like thermarest z rest) There are also newer style insulated inflatables (like msr neo air) they compress to the smallest I have seen (I have the regular size one I use for summer backpacking trips, but find it too narrow)
If using ANY sleeping bag, I would definitely use a liner (silk is best) to increase the life of your bag, as the oils from your skin, will affect the insulation making it not as efficient. A fleece liner will add 5-10 deg celcius rating to your bag as well.
If on weather sub zero, I would recommend a vapour barrier liner (VBL) it is specialized, but is essential for longer trips in sub zero temps. Vapour barriers are inside bag liners of plastic which completely trap all moisture emanating from your body. The theory is that once your body is saturated with a base load of moisture, your body will stop perspiring, and the moisture does not escape into the insulating material of the bag. Therefore no frost gets into the bag. Vapour barriers also give a 5-10 degree C extension of the temperature rating of their system. However for some, the feeling of sleeping in a plastic bag is not comfortable. I get around this by bringing 2 silk liners on an extended winter trip and sleep in the liner, then the VBL, then the sleeping bag.
Some sleeping bags have a water repellant coating on them which, if they are on a down bag helps keep moisture out. You can always pick up a bivy (gore-tex or other waterproof material or a waterproof over bag. However I find that sometimes in sub zero temps there is too much condensation and it freezes on the outside of your bag, and the inside of the over bag, unless you use a VBL.
Quality brands for down are: Marmot (my favourite, and they definitely stand by their product and warranty, 100% customer service rating IMO), Western mountaineering, Mountain hardware (though unsure now of quality since Columbia bought out I believe), TNF-The North Face (but only their high end low temp bags), MEC brand bags are good quality for the price (and fair temp rating,) I have owned or still own the above brands, and in some cases several. MEC down bags are actually really well made and have a rock solid guarantee (if unsatisfied take it back), they stand by their products and any product they sell for that matter. And they are ethically sourced (if one cares about that)
I also hear Sea to summit (Spark III is supposed to be fantastic)is good as well, though can not comment on them. I am sure others will and have chimed in on what they like and have used.
W.