Are you certain you want a -20? I would think long and hard on what your uses for it will be.
That said I own an older WM Puma with 4oz of overfill with what is essentially the GWS fabric. It is a very warm bag, I LOVE crawling into it during November elk hunts. Very high quality bag and funny enough weighs the exact same as my old NF 0* bag did.
But in all honesty it is probally way overkill, I usually dont have it all zipped up or I sleep in just my underware with it. I also have a 30* WM bag and it sees FAR more use. I usually find my self pushing the boundries of that bag rather than dragging the big Puma out.
Do you already have a nice light summer bag? Ive considered getting a tall 0* bag to compliment my 30, using the 0 during the fall/spring and then putting my current 30 in the tall 0 during full winter trips...
Several years ago I used to use a similiar system which worked well for me, but my bags werent the high quality bags I now own so the system weighed a ton.
Edit: Oh and of course if your considering the above companies give Feathered Friends a look too.
I would also consider feathered friends as well. I did a lot of reading on them when I was looking at down bags. Nothing but good stuff about them.
Forgot to mention why I would get kifaru or wm over the others. One word sums it up;quality! I have a marmot that I like.... But if I could do it all over again I would have gotten a wm bag for my really cold weather bag.
With a bag of that temp I would also get it with some kind of waterproffing, wm uses gore windstopper for theirs. The condensation that forms on your bag in those frigid temps is what will screw you
I would certainly go with the WM or the Kifaru based on Synthetic or the Down, buyers choice. Those two bags seem to be the toughest, hardiest bags of the brands you named off. The WM is a great high quality bag over the Marmot, JMO. I used the Helium and the WM Apache (15* bag )and went with the WM Apache seemed like a better quality bag. The Kifaru Slick (Syntheitic) would be the toughest bag I have used overall. I am somewhat diligent with my WM bag but at the $480 asking price I seem to handle with care when laying it out.
I do own the marmot helium. Used it at 12000 feet at 23 degrees and i thought it was pushing its limits. Just thought a zero degree bag is not much different. So i want the -20. I thought i saw aron mention the montbelle due to the stretching ability. But no one mentioned it. WM bags are dang expensive. Leaning towards kifaru.
One thing to keep in mind with the kifaru, is that they are not full zip. And especially with a bag that warm, you may get hot during those hotter times.
If i could go back, i would have never gotten my marmot lithium. I would have gotten the kifaru. I own a kifaru doobie that i use as a quilt to sleep with 3 season. Its amazing how thin and light they are compared to how warm they are. First thing i said to myself when i first got my doobie was no way this is going to keep me warm. But upon using it it has surpassed my expectations. The material they use feels tough as well! I have no problem dragging mine around where as my lithium i am very very careful with (especially in floorless shelters)
The synthetic kifaru dosnt hold moisture like down will, which i really like as well. And you can get them in multicam now
I have a 20* slick and a 0* slick bag from kifaru. I have yet to use the 0. My wife used it on her deer hunt a couple weeks ago.
My 20 got warm most nights.. and it was in between 20-30 every night and morning. If you get a 0* slick, you don't want a -20*, unless you are a super super cold sleeper...
Mine are longs (i'm 6'4"), only thing i'd say about them is i wish i would've got a wide instead of standard width cut. shoulder area is too narrow, but it's not uncomfortable, just too tight for my liking. width on feet and legs and body is good until you get to the shoulders, then its too narrow. that's my only gripe and complaint.
They are very true to rating and durable. I've used mine for about 25 nights this season now and it's kept me plenty warm and dry.
I was wondering about the slick bags. I am only 5"8. I am a little worried about heating the bag up. Since it is good to 6"1. How tall is Ur wife sneek-ee. And thanks for the bino harness. It works great....
I have the 20* base bag (slick bag that is compatible with the discontinued mob system). The zipper on mine goes down plenty far to hang a leg out when it get hot. I'm 5'11 and have the long bag and am glad but I like extra room around my feet.
I will chime in and give Mont Bell a nod. I bought the Super Spiral #1 bag this year and can finally say I am happy. The stretch in the bag is amazing. I can move around and the bag shrinks around me. No cold spots and the freedom to move around is great.
Also keep in mind that the temperature ratings that different companies use is different. For example, Western Mountaineering lists the European Norm "comfort" rating, so a 0* bag is truly comfortable at 0*, whereas Montbell uses the "lower limit" rating, so in one of their bags you might be ok at 0*, but the comfort rating is more like 15*.