Indeed this is all about keeping yourself warm at night IMO, unless you're the type that likes to take long breaks during the day. Even then, torso warmth layers are a lot easier to put on and take off than anything on the legs.
Assuming that perhaps you just didn't list headwear that you already plan to bring, i.e., sticking to the torso vs. groin/legs question, I'd definitely favor the torso. For one thing, you can do most or every in-camp chore with your lower body in the sleeping bag. And you already have long johns listed.
On the AT early last year the temps got down into the teens several nights. I used a WM 20F rated bag with down booties (highly recommended for cold temps, feet in particular can be an issue), a down parka, but just thin long johns and my normal hiking pants on my legs. This combo worked great for me.