To determine what's best for you, consider the following. In what conditions will you be using it? How much are you willing to invest? What comfort level are you willing to accept ? How long do you want it to last? Are weight and compactness important?
For consistently wet or damp weather conditions, consider either a synthetic bag (which insulates well when wet), or a goose down bag encased in gore-tex or gore-dryloft (and be careful to keep it dry! - down doesn't insulate when wet).
Synthetics like lite-loft, primaloft, polarguard, hollofil, microloft, etc. are superior for wet conditions, and are cheaper. That's about the extent of their advantages over goose down ! However, if you are primarily outdoors in wet, damp conditions, a synthetic bag may be the best choice. ( A good choice would be the newer Polarguard 3D which is less bulky and compresses nicely.)
Goose Down is lighter, more compressable, warmer by weight, and much more durable and long-lived (like 300%). With the invent of gore-tex and it successor, dryloft as coverings for down bags, down is a consideration even in damp environs. You can also further encase a down bag in a gore-tex bivy sack for greater waterproofing.
In the winter, some folks prefer synthetic bags for long-duration outings. The reason is that in extreme cold, your body releases moisture as you sleep, so the down bag gets wet from the inside even though well protected from the outside. One way to prevent that is to use a vapor-barrier lining which keeps the moisture away from the down.
Some manufacturers include: Western Mountaineering, Feathered Friends, Marmot, Ajungilak, The North Face, and Moonstone.
Attributes to Look For:
For colder weather, get a draft collar which cinches around the neck--keeps warm air in and cold out.
Generous draft tube along entire length of zipper.
For warm weather, look for ease of ventilation.
Full side zip so you can air out your feet during warmer weather.
700-800 fill-power down lasts much longer than cheaper 550 fill power. In the long-run its probably cheaper.
Double side zipper so you can still use the bag if one zipper blows out.
There should be a velcro or snap-shut closure over the zipper, at the top of the bag to prevent the zipper from sliding in the middle of the night.
Sleeping Bases / Pads / Mats
Closed-Cell Foam Pads, on the plus side, are ultra-light, inexpensive, waterproof, and durable. On the downside, they are bulky, inconvenient to pack, and uncomfortable for your body and the terrain.
Open-Cell Foam Pads, on the plus side, are ultralight, inexpensive, compresses better than Closed-Cell Foam and cushions well. However, the thing is really just a sponge. When it touches moisture it becomes a soggy sponge. Most often, the open-cell variety is encased in a nylon inflatable shell to protect it from the elements.
Self-Inflating Mattresses, are very comfortable, have adjustable air pressure, good body heat retention, compress better than closed-cell and, and are easy to pack. They are relatively expensive, are heavier than Closed-Cell pads, and are prone to puncture (optional repair kit adds even more weight to the pack).
In summary, Closed-Cell is lighter, cheaper, and bombproof. Self-Inflating Mattresses are more comfortable, compact, and warmer. To determine which pad is best for you, consider what your needs are. In what weather conditions are you using it (or a combination of them)? Consider importance of warmth, weight, price, bulk, durability, and general comfort. What's your priority ? You might consider a 3/4 length closed-cell for a quick minimalist over-nighter; or a full-length 1 1/2 inch self-inflating for a long-distance trail trek; or a combination of self-inflating and closed-cell during the winter (on the snow) for maximum warmth.
TWMC c/o John Payne
Tsukuba Walking & Mountaineering Club