At your weight you could do either. Weight ratings are really not that useful and are just guidelines for guessing a size. The length is all about stability and float, which is dependent on weight, stance, riding style, and personal preference. Riders with the same weight might want different sizes because one likes it more or less stable/smooth than another (or maybe one likes faster turning over smooth feel).
Given that you want backcountry and deeper snow I would find the general size YOU are comfortable with and go up a small notch. Also, if its mainly for backcountry/pow you might want to try the heritage or something more freeride oriented. If you also hit park, stick with the SL.