For years many have debated whether to head out with a down filled bag, or stick with synthetic insulation.  Both materials have their advantages and disadvantages and no clear cut winner can be found.  It really comes down to personal preference and budget. 

  Bags come in 2 distinct shapes.  Both will provide you with excellent comfort and warmth, however some prefer one over the other.

  No matter which material or shape you decide to go with, pay attention to the temperature rating of the bag you choose.  Too warm of a bag and you'll be sweating all night.  Too cold, and you'll be reaching for clothes.  Generally most trippers use summer weight bags in the 0 degree to 5 degrees Celsius range. 

• Down Bags:  Down is an amazing insulator.  For really cold nights, down bags will provide you with a comfortable slumber.  They pack down well and are incredibly lightweight.  On the downside, they are extremely expensive and will not hold they're insulating value when wet. 
   

• Synthetic Bags: Synthetic bags have come a long way over the past decade and provide excellent comfort and cold weather protection, all at a reasonable cost.   They will also hold their temperature rating when wet.  Over time however, synthetic fill will "bunch" and you'll lost some insulation and comfort.  They will not last as long as a well cared for down bag.
   

• Mummy Bags:  Mummy bags  feature a tapered shape and a more restrictive.  The idea is the less room inside the bag, the less heat required for your body to heat it up.  These bags are definitely warmer than barrel bags, but are less comfortable as they restrict your movement.

• Barrel/Rectangular Bags:  The bags of choice for most trippers, this style will afford you ease of movement in your bag, without the restrictions that mummy bags offer.  They're roomier and will take longer to heat up and usually won't compress as well as mummy bags.