
NEMO
Disco 15 Series
Spacious for side sleepers; bulky, temp rating optimistic.

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Not who you asked but I'd consider Kelty to be a lateral move from OP's current Coleman. Both make reasonable bags for car camping, but not anything I'd want to carry. Personally, I use a 32 degree Marmot bag for backpacking and it replaced(and is actually warmer than) a 0 degree bag from Kelty. The UL crowd really likes quilts instead of a traditional sleeping bag so that might also be worth considering. I'd also mention that the sleeping pad is a major factor in warmth so don't forget that. I bought a Neoair when they first came out and that made a huge comfort difference for me compared to the thin foam pads I had before.
Marmot is both heavier and bulkier as it's filled with 650 CUIN down while the RAB has 800 CUIN. But from the insulation standpoint, the Marmot has 1300 grams of down and RAB has only 700 grams. So Marmot will definitely be much warmer.
True, but not on this difference. The amount of down is almost double. https://rjtedge.github.io/downloftcalc/
Marmot has duck down ( all Marmot's 650 CUIN is duck down ) and RAB uses goose down for its 800 CUIN.
Sure, it's not that simple and it depends of baffle size , baffle construction and the overall construction of the bag, but in the end 1300 grams of 650 Fill down will be warmer than 700 grams of 800 Fill.
650 vs 800 fill refers to how many cubic inches 1 Oz of insulation occupies, ie. How "fluffy" is the insulation. Fluffier insulation (higher fill rating) will be warmer per ounce. However, in this case, there are MORE ounces of insulation in the Marmot so while not as fluffy, it will be warmer. Heavier... But warmer.
Western Mountaineering, Feathered Friends or Marmot for bags. Enlightened Equipment, Hammock Gear or Katabatic for backpacking quilts. Not cheap but quality and comfortable.
The Ice hotel in Montreal gives its tourists North Face and Marmot bags. They've seemed to hold up impressively to both repeated use and unprepared newbs. We weren't cold at all
I used to use a Sea to Summit Womens Etherlight Xtreme pad. It turns out it's just too cold for me. Even though it has been tested, I believe it's so thick that the air moves around when I toss and turn, churning up cold air. I tried a Neo Air Xlite. It's not comfortable for me, but incredibly warm. I settled on the Nemo Tensor All Season. I also have an Exped pad. While it's warm enough, it's not as comfortable as the Nemo. Make sure your pad is warm enough. If the cold is coming from the ground, it's the pad, not the bag that needs to be changed out. I found that I sleep about 20 degrees colder than most ratings. So, I use an Enlightened Equipment Conundrum 10 Degree for the colder weather. Then go to a 20-degree Enlightened Equipment Revelation. The Conundrum has a 3/4 zip, which allows for the warm air to stay inside the bag. It does not unzip the way, so it won't lay flat. I sleep with a buff on my neck and a fleece hat. I'm warm at 34 degrees, but I also need to use my fleece or puffy. At 25 degrees, I start getting cold. I have also tried a Marmot bag and an REI bag. They weren't warm enough for me. I believe Katabatic and Featheted Friends may use more down, thus making their bags or quilts warmer. They are more expensive than EE. In April and May, I can switch to my Revelation, which does open all the way. I would not use the Conundrum in zero degree weather because I sleep cold. Nor would I ever choose to be out in zero degree weather. I have a 40-degree quilt that I made for summer use. I sleep cold enough that I am uncomfortably chilled in it when it's about 60 degrees at night. Some people will tell you the EE quilts are not warm enough. They used to use less down but have changed it to match the standards that most others use to achieve the loft needed for a designated temperature. You should be able to see the fill weight on each manufacturers website. Pay attention to the width and length of the quilts or bags. It will help you compare apples to apples. Only you will be able to tell what will be warm enough for you. I don't like using a mummy bag, so the quilts work well for me. You could consider using a liner. It may help boost your bag or quilts rating. I think you will get more warmth for the ounce by getting a warmer sleeping pad. You will probably need to practice in your backyard or some safe place to help you determine what works for you. It frequently test my gear in my backyard, so if it fails, I'm steps away from the house. You may also want to consider purchasing from an entity that allows returns, like REI or MEC. I also sleep with warm socks. I tried Alpaca wool, which worked well. However, I found that using two pairs of Alpha Direct fleece socks works just as well. I bought one a size bigger for layering. This allows me to bring two pairs when needed or leave one pair home in warmer months. You have to look at your sleep system as a system, not as individual components. Combing my puffy, fleece, hat, socks, and yes, even gloves, help me adjust to different temperatures.

NEMO
Disco 15 Series
Spacious for side sleepers; bulky, temp rating optimistic.
Hammock Gear
Classic Burrow 20°F
Budget king; very warm but heavier than ultralight.

NEMO
Disco 30
Spoon shape for side sleepers; prone to zipper issues.

Western Mountaineering
VersaLite
Durable, very warm; premium price, too hot in summer.

Katabatic Gear
Alsek 22°F Quilt
Versatile 3-season quilt; secure pad attachment, high price.

Ranked #1
NEMO - Disco 15 Series

Ranked #1
Kelty - Tru.Comfort Doublewide 20
Ranked #1
Enlightened Equipment - Revelation APEX

Ranked #1
NEMO - Disco 15 Series

Ranked #1
Feathered Friends - Egret UL 20/30

Ranked #1
NEMO - Disco 15 Series