If you're a side sleeper, you might need to consider the girth and design of the bag. Some allow you to move and roam (Big Agnes, Montbell, some wider quilts), others do not (reportedly Western Mountaineering's mummies).
You can pretty much make a quilt of any bag if temperature is warmer than bag, but 10F bags are heavy, and you can get away with much less bag in the height of summer. Maybe a compromise between the two extremes with supplementary down clothing to push the envelope when needed would be "be$t".
It's a big purchase, so whatever you do, make sure to check out reviews here, (search google site:www.backpackinglight.com) and on trailspace, REI, etc., to make sure others liked their selection before you hit "order".