For the money that Marmot bg looks like a good deal. 600 fill down is medium weight down but will maintain loft better under mild compression (like a poking elbow or shoulder) better than higher fill downs. i.e. Lower fill downs don't compress as much when packing them.
I have a 30 F. WM Megalite bag with 850 oz./cu. in. down fill. It's good to about 25 F. in my experience, IF I wear light poly long johns and a light stocking cap.
"There are no comfortable backpacks. Some are just less uncomfortable than others."