Lost Ranger 15°

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 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. 

I think going all in like this is a bit much personally, but I do have a zenbivy sheet and quilt, and I found that for the specs etc. the value wasn't awful when I purchased, at least for backpacking equipment factoring in weight etc. For moto camping alone, it's a bit overkill. Breaking it down a bit: I think the pillow looks a bit overpriced, but if it's super good maybe not awful. The mattress also looks a bit overpriced to me - compared to a similar Nemo or Big Agnes. The dry sack is overpriced IMO, and not really necessary unless you don't have waterproof luggage. I just pack my quilt in wherever it'll fit to take up all the extra space. The quilt itself I think is pretty solid, 10° is pretty cold but maybe makes sense depending on where you're located, and the integration with the sheet is super nice IMO. The Big Agnes lone/lost ranger are competitive options that are priced generally similarly for context.

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.