Phil's Down quilt and Marmot Lithium. Lithium is EN comfort rated to about 10 degrees F. The Lithium on my scale is 2lb 13.8oz in a regular length. Phil's quilt is Phil-comfort-rated to about 20 degrees F and weighs 1lb 5oz. Either option is easily warm enough for winter when combined with other warm layers.
Phil's quilt used as additional warmth over the Lithium.
The Marmot Lithium and Phil's quilt equal 9+ inches of loft, about 6" of which is on top. This should, when used in combination, equal approximately -30 degree F comfort rating depending on how well you can seal out drafts with the quilt. This combination gives 3 bag options: The quilt for 3-season use, the Lithium for 4-season use, the combination for extreme cold. Total weight of the two bags is 4lb 2.8oz. Not bad for a bag that warm, and a heck of a lot cheaper than buying an $800 -40 degree bag you'll use twice.
Lithium vs. Down Quilt is size. Let me remind you that the Lithium is Marmot's ultralight, ultra-packable 0 degree bag. My Marmot Couloir, also a 0-degree bag, does not pack up as small.
When used in combination with the Rab Neutrino and Compressor pants, the quilt will easily reach 0 to -10 degree temps. The key to having a warm quilt is sealing out drafts!
The Lithium, in comparison. It's much larger when packed and more than double the weight. Most people will be taking a belay jacket and insulated pants on a winter climbing or backpacking trip anyway, so why take a heavier sleeping bag if you can use these as part of your sleep system? Some people claim they sleep better without having to layer clothing under a sleeping bag. I understand that, and that's fine. For the rest, think about trying out a quilt. It's a great way to lighten up and trim down pack size.