I'd get a bag rated to 15F. In general, females need a bag rated 10-15 degrees colder than males due to metabolism rate differences.

I have the Montbell #3. I like and recommend it, but it's not rated conservatively at all. In general, you can't actually sleep in a position which stretches the bag significantly because there will be a cold spot where your knee or shoulder is uninsulated. They work well unzipped and used as a quilt too.

I've considered trying the Big Agnes bags, but I'm a bit skeptical of drafts and cold spots with that design.

Look at the 15F 800 fill Marmot bags.