I have WM bags too; what sold me on them is their reputation for --- if anything --- understating their temperature ratings, and I wasn't dissapointed.

For a general use bag in the types of conditions you're talking about, I'd be inclined to err on the safe side and get a 20 degree rated bag. If you get one with a full length zipper, you can open it up in warm weather and use it like a quilt (that can be nice in not-so-warm weather too).

It's tough to estimate what the right bag is without knowing all the variables, including your metabolism (how warm or cold you tend to sleep), what kind of shelter you use, what kind of padding you use underneath, amount and type of clothing you have along that you're willing to wear in the bag, the range of conditions you could reasonably encounter, and how close you're willing to flirt with the edge of discomfort/danger for the most extreme cold nights.
Brian Lewis