What's the lowest temperature YOU'LL FACE?
This is where it pays to be a pessimist. Get a bag rated 5 to 10 degrees warmer than you think you'll need.
Which bag is warmer?
Temperature ratings are a good guide, but loft is better. Compare bags side-by-side; if the temp ratings are identical, put your faith in the fat one. If you sleep really cold, go 10 degrees warmer.
Do you toss and turn, or sleep like a mummy?
Bags come in different shapes. Get the right balance between efficient insulating (less space) and comfortable wiggle room (more space). For restless sleepers, a few extra inches at the shoulders (girth) can make the difference between snoozing all night and feeling straight-jacketed. Likewise, if you need less elbowroom, you can get a narrower cut, saving weight.
Down or synthetic?
Get down fill if low weight and low bulk are your priority. Tradeoff: If it gets soaked in the field, you're shivering. Go for a synthetic fill if price is more important, or you expect to see a lot of wet weather.
How many bags do you want in your closet?
For many hikers, nighttime lows can vary by as much as 60 degrees from season to season. You'll save weight in the summer and prevent cold nights in the winter if you invest in more than one bag.