I have and use both down bags and quilts. IMHO, A down mummy bag with the same amount of fill will keep you warmer than a quilt. Why, pretty simple really, a mummy bag will seal in more body heat.
I have a WM Megalite that I've slept into the mid-20's, with no additional clothing.
I mostly use quilts now, because of me being a side/stomach sleeper, and a tosser/turner as well.
I've used a down quilt rated to 32 deg, where I needed to wear a Capiline 3 top, bottom,socks and a Montbell Thermawrap parka, at, near, and slightly below the rated temp. of 32deg.
Note: that this was with the pad straps around my air matress and not around my body, which seals off more body heat, and is how the quilts are probably rated.
But, with the staps around the body, then you're limiting the quilt in regards to staying stationary and not moving with you, which is the best feature for me, this and the lower weight.