Down vs. Synthetic

• For most backcountry winter camping a down bag is the best way to go. Due to the extreme cold, winter camping below zero tends to be very dry, making down the logical choice. Down can be packed in less space, and is typically lighter than synthetic fill bags. Be wary, as down when wet has no thermal heat retention. If you plan to camp in areas where moisture or wet conditions exist, use a synthetic fill bag as synthetic fills retain heat when wet.
  
  

Packability

• Assessing the minus-20 sleeping bag requires looking at how small it packs. Space is almost always a concern on a backcountry excursion, and a sleeping bag is one of the bulkier items taken into the field. Check the packed dimension of the bag, found on the sleeping bag tag at the foot. If the bag seems to be too big, use a compression sack to compress it further. As a general rule, a down bag packs smaller than synthetic, giving down one more "positive check" in the best option column.
  
  

Shell Fabric

• If breathability is a concern, look for subzero bags that use a Gore-Tex or other breathable fabric for the shell material. These fabrics add additional costs to the sleeping bag purchase price but allow the body vapors and moisture to vent through the fabric, keeping the bag drier and water resistant. Using a standard nylon fabric shell is fine, provided you are heading into a dry and low-moisture area.
  
  

Cost

• Like most things, cost plays a large factor when purchasing goods. The best minus-20 sleeping bag may be the one that you have the means to buy. Winter sleeping bags are among the more expensive of the outdoor sleeping bags due to the extra material used in the making. If you have a budget, determine what the best criteria for your bag are -- fabric, fill and size -- and then find a selection of bags in your price range. Slide into each one and test the ease of use with the zippers, hoods and face guards and packability. Ultimately, go with the bag you can afford and the one that fits you best.