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There are two types of sleeping bags: down and synthetic. There's a lot of information about this on the web (see, e.g., this "Freedom of the Hills" excerpt), so we don't duplicate it here, other than noting that the Sierra Nevada are a relatively dry range, and a down bag is more appropriate here than it is in the Cascades, for example.
Other things to consider: what kind of material is the bag made out of? Some of the ultralight fabrics are not so durable, but they are light! Others have special water-resistant coatings (e.g. pertex, Marmot's membrain, ... ), which work to varying degrees (and add a varying amount of weight).
Down fill is rated by loft, as in 650 fill, 700 fill, 800 fill, 850 fill. The higher the better. Even though the loft is intended to be an objective measure, it doesn't quite give a 100% indication of how warm the fill is. Brands with good track-records of using high quality down are: Western Mountaineering, Feathered Friends, Marmot, Patagonia.
- An excellent 2010 Supertopo Down Sleeping Bag review covering high-quality sleeping bags in the 15 F to 25 F category.