Riff

I'd go with down, honestly. I've backpacked in some extremely humid areas and have never had a problem. Nemo Disco 15 or Riff 15 would probably do you well. REI Magma 15 is great too.

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.

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.