Setting aside your metabolism and how it changes during the course of a multi-month hike (a dissertation in its own right) the environmental reality is the bag you're using mid-summer isn't the bag you want early and late in the trip, and vice versa. If one bag has to serve the entire hike then yes, a 20-degree bag is a safe choice and in that case, probably the gold standard is the Western Mountaineering Ultralite.

Brian Robinson, who was first to do the "calendar triple-crown" of all three major N-S trails carried a Feathered Friends Hummingbird. I'd have no hesitation recommending either.

Cheers,