Just to put my two pennies in here, and they are very inexperienced pennies, I've used Snugpak for synthetics, Alpkit for Down and currently have a Wiggy's ready to go for Finland this year.
I'm fairly sure Snugpak overrate their bags temperature ratings. This isn't to say I don't like them, but I have found the chill beginning to permeate through the bag when it approaches 0 Degrees C, when my bag is comfort rated to -5 Degrees C. I love the bag, both for the relative lack of bulk, and a fairly low weight. Added to this, I've now had two British Army bivi bags fail on me (one time I awoke to find myself tangled in the seam tape!). Both times I have only noticed that my sleeping bag is wet after I pulled it out of the bivi bag. Last weekend the foot of my bivi bag leaked water into over an entire night of abysmal weather. I had removed my sleeping socks because the temperature was higher than I had anticipated, and still wasn't even slightly aware of the moisture till I packed away the next morning. Moreover, when I previously used the Snugpak Antarctica RE last year in Finland, in a bivi bag that was so unbreathable in those temperatures as to having the dry the inside of it over the fire, the bag kept me warm for a the whole week, despite having alot of residual moisture in it. I can easily imagine that in both situations down insulation would have been royally scuppered.
My short stint with Alpkit didn't reveal much as I soon moved it on to someone else, but I definitely noticed a drop in temperature rating after a night of heavy sweating. I aired the bag out all day, and found it to be far less warm the following night. Of course, this could easily have been a factor of food and energy as I'm a fairly variable sleeper (although leaning towards hot). I'd love a down bag for weight and bulk, but I'm very cautious of it.
As for the Wiggys (an Antarctica lent to me by Gray), I've had about 20 minutes in it one chilly night before I had to switch back to my Snugpak. It was so ridiculously warm. I only lasted the 20 minutes because it was so comfortable. It acted like a duvet, draping across the body. If bulk were not an issue I'd be more than tempted to buy my own and cart it everywhere. I'll know more after January, but for now I'm fairly certain it would see me comfortably to very cold temperatures.
I think if you wanted reliable warmth, I'd go synthetic (or a dual bag system with a mixture of synthetic and down), and I'd lean towards Wiggy's for comfort.