If it helps any-
So far in my hammock experiments I've found the bottom quilt to be the critical piece. On the ground I can "push" insulation much farther than in the air.
If money is tight and you have to choose- a good underquilt would be my choice. And deal with whatever you can afford up top or wear extra clothes.
If anyone knows what's in it, I can give you a rating. But generally I hear 50* average on those.