Neutrino Pro 500

I’ve owned a couple of those Mountain Equipment Helium range sleeping bags and have been very happy with them. The 600 is the bag I tend to use for midwinter camps, and it has a great warmth to weight ratio. I wouldn’t say they shed feathers any more than any other bags I’ve owned (I’ve been camping for 35 years so I’ve owned a lot!). Between the Rab options, the Neutrino will probably be slightly cosier as the Ascent is a slightly roomier bag. I wouldn’t personally buy the Robens bag as 600FP means it’ll be a bulkier than the others. I personally just don’t buy any sleeping gear with less than 700FP. You mention needing a three season tent but I’d say these bags are all more winter oriented. Even in Scotland, where temps are a little lower than England, I’d say these are potentially overkill for three season camping. When I’m camping in Scotland outside of winter I actually often use a quilt or just my Sea to Summit Spark -1°C bag (I was actually using the latter just a few weeks ago in the Highlands). What sort of terrain are you usually camping on? Do you have any particular preferences for certain types of tents? You mention not using trekking poles so a trekking pole tent wouldn’t be an option but do you have any other particular preferences like free standing, or large vestibule, etc?

The Rab Neutrino Pro sleeping bags are excellent, and there's some great prices out there currently since Rab recently updated their range.

The Rab Neutrino range are fantastic.

The Rab Neutrino range is an awesome sleeping bag.

Will you always camp together? We went to the Rockies for our honeymoon and did loads of Backcountry hiking. We used it as an opportunity to buy a double quilt (we got a thermarest one but enlightened equipment also do a brilliant one) and a double air mattress (although now there are good ones where you have two the same and can attach them together.) all together with a silk sheet it's lighter than one person's sleeping bag and air mattress and its so so good it's unreal. Way better than two sleeping bag and because you're in it together you stay way warmer than you do by yourself. I cannot recommend it more highly. Big Agnes do a range too, they're a bit more fancy with more bells and whistles but I have heard they are good. Sleeping bags, Rab neutrino pro range.

Strongly disagree. Fill weight does not equal fill power. As another user commented, one rates by safety and the other by comfort. You won’t die in the marmot at 0 degrees but you won’t be comfortable either. The Rab rating says you’ll be comfortable down to 5 degrees but the safety rating is likely -5/-10.

Ran is top tier. iirc all of their stuff sacks are woven onto the inside of a dry bag and that attention to detail comes with a slightly higher price.

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.

Yep. Loft (higher fill rating) is how high end bags reduce weight.