Buy the 0 degree one. You'll never notice the 6 additional oz. and if it's too warm at times just open it up or lie on top till early AM when it gets colder.
At my last physical two years ago (right after I quit smoking) my doc. gave me a three page document listing all the shit in cigarettes. It's truly amazing I'm still alive. Unbelievably nasty shit those cigs. Just quit...if you have an urge to smoke, do what I did...masturbate