This is a good question. For me, the decision is easy for the extremes: In Cold/Dry weather, where the chance of getting the bag wet is small, then I almost always go with a down filled bag. And go with higher quality fill (this means a 700-900 fill). A 500-700 fill has less quality but is also cheaper. Down bags are more expensive (1.5-3 times) than synthetic fill, but they last longer, weigh less (especially as the amount of fill goes up the weight saving becomes more significant). In wet/warmer climates such as coastal brown bear, coastal goat hunts, Sitka Blacktail, Roosevelt elk hunting, etc. then go with a synthetic fill bag. Also, when using a bivy bag in damp climates, use synthetic as condensation can be high. Down does not do well with condensation. The "in-between" situations depend on the person. If you use a high quality tent, carry your sleeping bag in a waterproof stuff sack and are careful to keep the bag dry, then a down bag is usually my preference, for the weight and space savings it allows. If you want to just throw the sleeping bag out on the ground, use less than ideal tents, etc., then I'd stay with synthetic. As far as size goes, I use one size larger (longer) than what is recommended, that way I can keep my boots and gaitors (in a garbage or leight wt. waterproof bag), in the foot of the sleeping bag, to keep them from freezing in sub-freezing weather. The choice between a snug/small mummy bag, a wide mummy or something in between is personal choice. I prefer the med sized Mummy bags as they give me more room, but are not too much heavier than really narrow one. Also, keep in mind that a sleeping pad is nearly as important as a sleeping bag for warmth, sometimes more so as they insulate you from the cold, frozen ground, which causes more heat loss than from the air. Also, sleeping with a warm hat, long underwear and socks will help increase the warmth of your bag and will keep it from smelling like a sweaty mountain man, that much longer. Sleeping nude in a sleeping bag, unless you have someone in it with you, is not warmer than sleeping with some clothes on. As far as makes, models go, I've had good luck with Integral Designs (down or synthetic), Western Mountaineering (Down), Feathered Friends (Down), Montbell (Down or Synthetic), Marmot (Down or Synthetic), Wiggy's (Synthetic), Kifaru (Synthetic). The company is not as important as the features, design and quality of materials used. Look for a good zipper, with stiff material around it, so that it does not catch, a good hood and shoulder fit for those cold nights and a reasonable warmth to weigh ratio. One an African Self guided hunt in Cameroun, I just took a $60-80 fleece sleeping bag and a sleeping pad; this combo worked well as night temps were 50-60 F. After you finish your trip, let the bag and sleeping pad air out well before storing. Store sleeping bags either by hanging in a closet or in a loose/large sac (most high quality bags come with one) or in a big duffel.