Don - Thank you for recommendation, I will have to check out the Montbell :)
Walter - I had checked out the EMS offerings as I saw a 0 degree when I was there last week and it seemed like nice construction. My only hesitancy was that I see a lot of mixed reviews on their bags. While I obviously try to save money wherever I can, I don't mind paying extra for quality, especially with such a critical piece of gear as a sleeping bag for sub zero temps.
Robert - Thank you for the recommendation. The Puma had come up quite frequently in my research. Right now it is a close second for me. I'm leaning toward the Marmot though, not only because of my history with their products, but also because of the Membrane fabric as I'm worried about condensation in a bivy. While I am very experienced at summits and day hikes in the Whites in sub-zero temps, I have never slept out below zero. I have definitely pushed that mark, but never really ventured into winter alpine sleeping conditions. If I want to dry out socks, base layers, etc in my bag overnight, I worry about condensation in the reduced volume of a bivy. Have you had any issues with this in the Puma?