I have backpacked in past winters with my TNF Darkstar and since my winter pack for overnighters is a Kelty RedCloud 6000, I can fit near everything into it. I compress the Darkstar which is a -40 degree bag and weighs 5 pounds and it takes up a little under a 1/4 of my pack, which isn't bad for a -40 degree synthetic. I have only onwed synthetics in the past, 3 TNF bags, a Cat's Meow, a Snowshoe and a Darkstar and the Cat's Meow and the Snowshoe both work pretty well but I haven't been pleased with my Darkstar, it didn't work below -10 degrees.
Having said all that about my synthetics I must say that I bought a Western Mountaineering Highlite, a down bad rated to 40 degrees above 0 and I used it on the Long Trail and I am a WM convert. It is one sweet bag and kept me warm actually toasty warm at the tent platforms outside Cooper Lodge which is almost at 4000 feet without a tent and without extra clothing. I LOVE that bag and it squishes down to the size of a football. I am going to sell 3 of my 4 other sleeping bags and invest in 2 more WM bags. I am going to keep my TNF Snowshoe because I really love that bag.
I say go with down unless you have something against down. I used to be worried about the down and wetness thing but after numerous torrential down pours on the LT followed by cool nights, I am convinced that careful packing can avoid wet sleeping bags. Anyway, totally a personal choice, it has taken me years of backpacking before I switched and I am a down bag convert now.
sli74