I've got the sb20 generally good, packs a bit bigger than a cantalope. Good for me till 20s with a pad and dry base layer.I use the SB32. 800 fill. I think its 1 step up from cosmic. Works great temp rating is a tad off. Im thinking about going with the 20 and that would be perfect for low 30 degs.
Bag would be used mostly out west Colorado, Idaho, Montana. Using a tent with floor for now. Months of late Aug and into October.
I'm new to mummy style bags so trying to figure out fit. I've heard if bag is too big you lose some heat.
I've heard synthetic dries faster but salesman at REI stated treated down is as good and would dry out faster.
All that being said he's a hiker and not back country hunter.