King Solomon

Keep an eye on geartrade.com. My husband and I recently got a synthetic 2 person big Agnes Cabin Creek sleep system for $175! It was in excellent condition but an older model. For more money than your budget, try the down Big Agnes King Solomon (on mega sale like today!) For less money than that one, but more weight, try the synthetic Big Agnes Dream Island. There’s also the Zenbivy beds but they’re pretty expensive and a bit heavy.

Big Agnes king Solomon. Been using one for years. With 2 20” pads it makes a super tight fit. Warm, light and worth every cozy night. Backpacked 30 miles with it no problem.

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.

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 would go with a quilt, they are great for moving around. Zenbivy is most peoples favorite, I really enjoy my Big Agnes.

If your biggest concern about a sleeping bag is room for your legs to move without feeling constricted, I would try out Nemo or Big Agnes sleeping bags. Both tend to have wider profiles. Definitely go for a bag that is rated at least 10 degrees lower - preferably lower - than the temps you expect to sleep in. Theres multiple problems with layering with clothing like this, one of which is not the warmth specific factor but the breathability. One or two layers is one thing, but with all the listed clothing and then something like the picture on top, you’re going to build up humidity near your body you can’t get rid of, and you’ll become even colder. Next, a sleeping bag basically creates a warm pocket of air around you. The insulation in the bag helps create and maintain this pocket. The more compressed this insulation, the worse it performs. With layering clothing, you compress the insulation, making it less effective. On top of that, insulation is minimal if not nonexistent at the seams. Some puffy jackets and “extreme temp” clothing gets around this by minimizing seams but there’s almost always a few somewhere, which means no air pockets and no warmth. If I could recommend the best case scenario for you, it would be this: find a sleeping bag you can tolerate rated to a temp appropriate for your conditions, 15F at a minimum, best scenario a 0F. Dress in only your smartwool layers, and make sure you have a good pad/sleep set up underneath you. Sources: avid outdoorsmen, backpacker in sub 20 degree temps, and former REI employee who specialized in camping gear.

A Big Agnes sleeping bag with down that fit me correctly. I was iffy on the price at first, but it turns out I had basically used ill fitting, non insulated sleeping bags for years of wilderness camping. Getting a good nights sleep anywhere, especially away from home, makes a huge difference.