Lost Ranger 15°

Having tent camped for years, then hammock camped for a decade, and recently gone back to ground I can honestly say this: quilts for hammocks, bag for tent. Quilts are too much work with a sleeping pad. You are better off getting something like a Big Agnes sleep system bag and pairing it with a pad. Much easier, and less drafts

I have been using a Big Agnes down bag. I think it was in the “ranger” series. They change models every few years. But stick with the “sleep system” bags. They don’t have a bottom side similar to quilts, but they stick to your sleeping pad much better, and wont let drafts in when you move around. Only issue with their bags are the temp ratings are not the best. But I mainly tent in warmer temps and save the hammock for really cold winter temps. (I went back to ground because I finally got a dog to take on adventures, and I want him safe from mosquitos and what not at night just like me)

I have the Big Agnes Lost Ranger UL 0, which is a hybrid quilt-like solution, but with an inner sleeping bag. The inner bag is rated for -2C and weighs 850g. You could keep the outer bag for less cold nights (around 5C) and the whole setup for those really cold nights (-11C). It’s a bit pricier than usual bags but it’s so versatile and they are actually 3 bags in 1.

We have a Big Agnes something or other 15 degree down bag we bought for around $300...10 years ago. It's great, although more like a 30 degree bag. Size / weight is about equal to two sleeping bags. We also tried out Feathered Friends' Spoonbill which is well out of your price range but might be interesting to look at to see how an actually warm two person bag looks.

 So the problem with any normal two-person sleeping bag is that there's an air gap between you if you don't sleep snuggled up.  The Big Agnes we have has a small flap that covers that up. We have also stuffed clothes in between us when it was colder. That works for 15 to 20°, but not colder. The reason I said the spoonbill was interesting is that it has individual draft hoods for each person so you share warmth but actually can't cuddle. 

I read the comments about enlightenment equipment, then went to the site, damn, they are too expensive. I looked for a quilt I would like, 1000$ no thanks. Personally I have Big Agnes and they are high quality, in my experience, and cheaper. I have a -15 degree down sleeping bag from them thats going on about 12-15 years. If my bag falls apart, I'll just stuff it and sew it back up.

I have a Big Agnes lost ranger UL which is a 2 part system, one of which is like you describe, the other a regular bag, or you combine both for maximum insulation. 

These were all over the industry a couple of years ago, but have fallen out of favour, and I actually can't find any current single-bag, adult sized offerings by a major company that are made this way. The Rab Mythic Ultra 120 "modular system" is now discontinued, I believe, but would be the most UL uninsulated bottom option. The BA Lost Ranger is an actual modular system consisting of two bags, a fully insulated mummy bag and a backless/pad sleeve style outer rectangular bag

Since it's currently at full price, the Magma is about $100 more expensive than either of OP's choices. The big difference is that has 800 fill power down, where the other two are 600 and 650 fill power. My daughter recently replaced a synthetic bag she bought while in college with an REI model, and liked their new sizing system. I think she may have bought the REI Magma. I have and like a Big Agnes 650 down bag I've had for years, but at a 15 degrees F rating, it hasn't gotten a lot of use in recent years. I agree with u/TheBimpo that, unless they are trying to put a full set of gear into a pack of 50 liters or less, OP will do fine with either of their choices.