Right on. <blind sage asks student> Which weighs more grasshopper, a ton of down feathers, or a ton of synthetic fill?

You have asked to compare a 55 temperature differenential. If a (least temp) 35 degree down bag would work then why would a 35 degree synthetic bag not work? I think that if a 35 degree down bag would work then a -20 degree bag would not. It would be too hot and heavy to justify.


Pick three
light
warm
cheap

Light - this is a viable choice that would have to be considered in bag selection.

Warm- does not appear to belong in this scenario.
A 35 degree synthetic should be as warm as a 35 degree down.

Cheap- Obviously part of the selection process, but that could be subjective - If a 400 dollar down bag lasts twice as long as a 200 dollar synthetic bag- theoretically they cost the same. And the benefit would be in weight savings.

I think warm should be replaced with some other category, perhaps, durable. Humbly, IMO