There is a MAJOR difference between camping at -20*F and -40*F, believe me. The former is the lowest edge of the temp. gradient to +15*F which is where the best winter camping IS, as it is DRY and cold enough that a good down bag will really perform.
-40*F and colder is just brutal and I have spent enough time winter camping, oftimes alone and bush working in such temps., that I advise people to AVOID such extremes if at all possible, the discomfort and danger is seldom "worth it". I have camped at a measured -41*F at the bottom of Kokanee Glacier in the West Kootenays of BC; I did this with a Swedish "Caravan" double bag piece of over-rated, over-priced crap and also my original Marmot custom down/GT bag....I won't do it now as I was young, tough and a working pro in the mountains then, circa mid-late '70s and am old, soft and SMARTER now!
For use in extreme cold, my FIRST choice would be an Integral Designs XPDIII Arctic bag with the 900 fill option, and Endurance shell, standard. I would definitely use my ID VBL with this and two EVA pads plus an inflatable insulated pad underneath, EVA, Ins. Pad, EVA, is the way to go here.
These bags come in various lengths and widths, the quality is absolutely top notch and they are NOT cheap. ID will sometimes do custom models of this, it just depends on what Evan wants to do at a given time...as he told me a few months ago.
My next choice would be a Western Mountaineering Bison, probably with "overfill" and the WM "Hot Sack" VBL. The Feathered Friends Snowey Owl and Snow Goose also fit in here, and they are also NOT cheap.
However, for MOST winter camping in western and northern Canada, I have found my ID XPDII Himalayan custom OF to be often MORE than enough and this past year went to the incredible Valandre Shocking Blue bag, a bivy or cover with it and my VBL, if indicated. This bag weighs THREE LBS TOTAL and it will work down to -20*F if set up right....cheap, well.........NO really worthwhile winter backpacking bag is "cheap".
Quality?
Valandre-ID, about equal, but, Valandre is ahead on design. These two makers supply the finest bags I have ever used and, IMHO, the price is worth it.
WM, superb, very restrictive in repect of sizes and widths, a smidgen less bag than an ID or Valandre, but, the price of their microfber shelled bags is VERY low for what you get and they are the best deal around.
FF-MINE, a 1989 model is excellent, however, there have been recent reports from sources I consider valid which detail some less than satisfactory incidents. To my eyes, the newer WM bags are superior, but, FF may be getting back to their once pre-eminent position in the industry.
Finally, you might look at Peter Hutchison Designs of Merrye Englande; they have an outstanding rep, offer total customization on their website and have certain features I find very appealing on their "Xero" models. I would love to try one in January in the BC interior.
These are the bags to choose from, IMO, study the various websites carefully and consider your body type and how you sleep. I would go with a -30C bag as the "coldest", UNLESS I were doing polar things or living out all winter in the Yukon or NWT or Alaska's North Slope. My -30 rated bags have ALWAYS been warm enough for any uses I put them to and a true -40 bag is not very useful as it is HOT at just over 0*F, something to consider. HTH.