I've been gone for entirely too long! I have been using quilts for most of my backpacking trips but have plenty of sleeping bag time on regular camping trips. The quilts i've used ranged from 56" wide rayway style quilts to Fornshell SUL tiny quilts in conditions down to single digits. Aside from off topic things like cuddling if possible, using an adequate sleeping pad (it's lighter to use a full length, warm pad than to have to have more insulation in your bag/quilt to compensate. Also, pad warmth is lighter than crushed insulation warmth, period.), or using a shelter that adequately blocks rain AND WIND from your sleep system as any good shelter should, I have come to my personal conclusion as to the best insulation for most UL backpacking- a topbag.
In my experiences with all 3 insulation systems, all other variables being equal, a topbag with a 1/2 length zipper, drawcord, pad straps and adequately warm hat solves most of the problems of both quilts and sleeping bags.
Weight and Volume- toppbags are lighter and pack smaller than sleeping bags of equal warmth, and quilts of equal warmth (not top insulation weight, but warmth). This is because they have the right insulation in the right areas (high loft on top, pad on bottom), require far less top insulated area than a quilt, along with blocking drafts far more effectively. The large amount of warmth gained over quilts from blocking drafts allows a topbag to have a lower weight of insulation for the same warmth unless the quilt is very large in size with a great seal around the neck. No drafts also means no bivy, sans bugs.
Ventilation- I still haven't quite figured out why quilt advocates can't concede that it isn't hard to unzip a sleeping bag and flap it to ventilate. It is easier to ventilate with a quilt than bag/topbag, but any bag with a zipper should vent well.
Drafts- As mentioned before, topbags do a great job stopping drafts. Drafts were my main reason for converting my quilts to topbags. After the conversion, i was immediately able to tell a significant warmth difference from their former selves because of this.
Tossing and turning- pad straps keep a topbag in place on top of the pad so you can roll around inside and don't have to worry about rolling off your pad like in a sleeping bag. They also prevent the fabric from shifting and coming in contact with cold air. there isn't as much freedom of movement as with a quilt, but you can move your legs around and curl up. I have found that hooded sleeping bags with straps on them won't let you sleep on your belly without a face full of hood, so i keep mine hoodless.
For my 11 oz summer quilt that was too small to block drafts at all, I added a 3D footbox to give me more coverage. It still didn't work well enough so I added fabric on the bottom and pad straps at a cost of about 2 oz, with a zipper and drawcord coming soon at another 1.5oz. Not as light, but much warmer and actually usable.
My quilt with 6oz of Ploft which was adequately sized for a quilt got trimmed down a lot, a 1/2 length zipper, drawcord, pad straps, and fabric on bottom. It is now warmer, packs smaller, and is about 5oz lighter, IIRC.
Downsides: cost, availability, still has claustrophobic feel like sleeping bags, can't unzip to be used like a quilt.