I can only speak from my own experience.

I used to own a high quality synthetic Qualofil bag. The bag was huge and heavy and developed an odor only I could tolerate. After 20 ++ years of steady use, and cleaning, I pitched my synthetic bag in the dumpster this last spring. It was a sad day. Hopefully the newer synthetics are lighter and more pack friendly.

I'm left with three down bags, a 15 degree, 40 degree and a 0 degree. The only bag I've used in the last couple years is the 40 degree bag. I now use the 40 degree bag with an Exped down mattress and I stay warm at temps below 30 degrees - I sleep like a rock.

In all my years of bumble headed adventures -I've never had any of my down bags get wet or so damp I couldn't stay warm. I should note that I always travel with a good quality leak proof tent and waterproof panniers. If I toured exclusively in wet weather that constantly hovered around the freezing point, in which it was constantly raining every single day...I would bet you I would have a synthetic bag or I'd quit outdoor sports. As it is, I don't tour in those conditions.

Remember - the bag is part of a system that includes your pad and your tent.

I'll post a picture of my Thermarest 3/4 length pad, Exped full length 3"+ thick down mat, and the 40 degree Marmot down bag.

For comparison:

Two each, 3/4 length, thermarest pads + protective bag weigh 1 1/2 pounds

Full length Exped weighs 2.5 pounds

Marmot Atom 40 degree bag weighs 2 pounds

See pictures - Good luck. Mike