Riff

My advice: NEMO women's Disco 15. First off, sleeping bag temps (ISO/EN) are rated for slight discomfort. If you want to be mostly comfortable at 30F, you get a 30F bag; if you want to be warm and toasty, you get a 20F bag. If you think temps may drop into the 20s but may realistically be 30s, you should get a 20F bag; if it hits 20F, you're fine but you'll otherwise be toasty. Women's bags are rated at the Comfort rating which is toasty and nice for a warm sleeper, slightly uncomfortable for a cold sleeper - which is the assumption for women. Also what it sounds like you may be: a "cold sleeper"/someone who likes to be warm at night. Know that you can - and should - try other gender bags for fit as you may find better fits across genders. But keep in mind there is ~10F temp difference between men's fits and women's fits (men's will be colder for cold sleepers, women's will be warmer for warm sleepers) and so don't just trade 30F for 30F. In your case, use the "Comfort Rating" as your slight discomfort temperature and adjust from there. Know also that this may work against you price-wise; they do not price based on fill, they price based on advertised temp. So a women's 30 will be the same warmth as a men's 20, but will be priced as a 30, not as a more expensive 20. So the Magma and the Disco may not be warm enough for what you're looking for. Of the listed bags, they'll fit in order of how you listed them: Disco will be the widest, then Big Agnes, then the Magma. The Magma is a true backpacking mummy and won't be ideal for a side sleeper. The Disco has extra space in the shoulders and hips which are ideal for getting on your side in the bag. So for a cold sleeper who sleeps on their side and is likely to have some 20F but maybe mostly 30F nights - NEMO Disco 15. Should check all of the boxes. The one big difference between the NEMO and the Magma is down fill power, which also impacts compressibility and weight. Magma is an 850 while the Disco is probably around 650-700. If you want a similar fill power bag go with the Riff 15. 850 fill down also warms up quicker because the feathers are smaller. It will also probably be more on par price-wise with the Magma. The Riff has all the same features as the Disco though may have a slightly slimmer shape. If you can, go to your local REI and get in each one to see which are comfortable and which are not. But based on the info you've given, I'd suggest either a Women's NEMO Disco 15 or NEMO Riff 15.

As a warm sleeper I would absolutely die in a 15 degree bag if it's 30+ out. I would have to fully unzip it and then struggle to regulate temperature all night. The Nemo gills help a little but they do not help enough if you're a warm sleeper and it's 15 degrees over your bag rating. I have a Nemo 35 degree bag that I use and I've never been cold in it even though the comfort rating is 40. If it's 50+ degrees out it gets too hot. 35-50 is definitely the sweet spot for it. Personally, as long as you're not in dangerously cold conditions, I'd always go with a warmer bag and wear extra layers to bed or have a backup quilt to throw over it in case it actually gets too cold. I've had way more camping nights ruined from having a bag that's too hot than one that's too cold. You can always add layers but once you have to fully unzip your bag because it's too hot you're in a tricky place with temperature regulation for the night. If you're a hot sleeper do not buy a 15 degree bag if you're basically only camping above 30 degrees. I would be looking at a 35 and then also buying a cheap synthetic camp blanket as an extra layer when needed or an only layer for nights that are 50+

Cheap, warm, or light and packable - you can only have two, you’re never going to get all three. You’ll also be colder in a bag for two. The more air space in the bag, the harder it is for your body to warm all that space. So the colder it is, the less comfortable that will become. Mummy bags are designed to be conforming to your body for that reason. They are not meant to be zipped together. So a pair of rectangular bags will fit together but be bulkier and harder to be warm in. Nemo Forte, Disco and Riff bags will zip together and be warmer- but they will also not be cheap enough for you unless you get the heaviest versions. Synthetic also breaks down faster than down. They are a compromise between mummy and rectangular. Then there is the pad issue. Two single pads tied together will have a cold gap in the middle. A two person pad weighs a lot and takes up more space in a pack. Two rectangular pads will do better than two tapered pads, same trade offs. Everyone has a different equation for what is important. But it’s hard to sell the gear you no longer want unless it’s of sufficient quality to be desirable. Higher end sleeping gear includes Western Mountaineering, Feathered Friends and the top tier versions of Marmot bags. if your boyfriend is already backpacking, he has cheap stuff. You say that he doesn’t have good stuff. I highly recommend that you start with moderate quality gear like Marmot or REI brands and get separate bags because you don’t even know how much you will want to backpack. He already backpacks and will want lighter and warmer. Get couples gear when you are sure you will want to go often with him.

I bought one Riff for myself and ended up giving it to my son and I bought another one for me. I also got both of mine for around $200 each brand new. I really like the spoon shape.

Fellow rotisserie here. I probably move in my sleep more than anyone on the planet. My ex said I do cricket jumps in my sleep. So I think there are kind of 2 philosophies. Either you get a quilt, which sort of naturally solves the issue by not restricting you, or you ironically get a mummy bag and perhaps being so restricted will inhibit you from moving as you normally would and sort of force you to chill out. It’s going to vary by person. Third option for people who have problems with quilt cold drafts due to their movement - wide bags. The problem with wide bags is they’re going to be harder to warm at a given weight or heavier to loft at a given warmth, they’re basically inefficient thermally. That said, Western Mountaineering makes some high quality ones like the TerraLite. I personally use a Nemo Riff. It’s what they call their Ultralight Spoon shape. Wider at elbows/knees than a mummy, but narrower than their Relaxed Spoon shape of like the Disco and cheaper models. I got really lucky on it for $200. Outside of a backpacking context or if weight doesn’t matter, just go with a rectangular bag.

So the Bantamweight is not a wide bag at all, it’s a mummy bag. I have a Nemo Riff that I really like, they call it a spoon shape iirc, wider at knees and shoulders for easier tossing and turning. I got mine at a really killer deal for $200. The Disco is an even wider cut that's a bit cheaper and uses lesser fill power down. Wider bags do sacrifice warmth:weight ratio, no getting around that. Have you considered quilts?

You can def try on r/ULgeartrade. People are often selling there for reasonable prices and gently used. Quilts are better for low temps above freezing and excellent for warm nights because you can kick a leg out or let air in. I prefer the ones with a closed/sewn footbox for colder weather, but a drawstring is fine for warmer weather. I have a nemo riff 15F as well. The 30F custom quilt is rated for comfort, while I would say the Nemo is rated for survival and they felt about the same on 30F nights. Quilt is much lighter. I like a quilt that has a pad strap system that allows me to pull the quilt under my body and keep it there even if I turn. HG Burrow is around $300. Haven't used it, but I have a UGQ 30F long/wide and love it.

As you've already alluded to, there's a difference between sleeping bag comfort and survival ratings. The Nemo Riff 30F is limit rated to 29F but comfort rated to 39F, which means you'll likely get a bit cold but remain safe if temps drop into the mid 30s. Whereas the Riff 15F is comfort rated to 28F. If you expect temps to regularly drop into the mid 30s, I would go with the 15F version. You can always open the "gills" or unzip the bag on warm nights, or even use the bag more like a quilt.