If you want a little more flexibility to ride out colder temps, I'd recommend you go with the 20*F quilt rather than a 30*. Depending upon the quilt design, you can always open the quilt up more or cinch it down to adjust for temps.
A few other thoughts...
Width/cut... Selecting a quilt of appropriate width is really key. A quilt that isn't wide enough to allow for a good wrap around your upper torso/shoulders will allow for drafts. This is no fun as temps drop. Different quilts manufacturers also provide different cuts through the torso/footbox. Some like more room to spread out, others like a slimmer cut for thermal efficiency and weight savings.
Footbox design... I'm on the fence. I have two quilts with a sewn footbox (Katabatic) that are super warm and cozy. Problem is, when it's a little warm out for the quilt, you can't easily vent or otherwise open up the footbox other than just hanging a leg out. Other designs, like the EE quilts, allow you open up the footbox for warm(er) weather use. On a dedicated summertime or warm weather quilt, this can be a nice feature. For a 3-season quilt, it might not be as ideal since it might not be as warm as a permanently enclosed footbox.
Strap designs... I really love the Katabatic Quilt strap system to attach the quilt to my sleeping pad. For a rotisserie sleeper (I spin round and round in my sleep), this system successfully keeps the pad under me and the quilt on top of me.
Hood... Definitely key as temps drop. Or plan to wear your down parka inside your quilt.
Layering... So, I have a 30*F 3-season down quilt and a 15*F down "winter" quilt. Earlier this year I purchased a 50*F synthetic quilt to use for the occasional really warm trip and to extend the range of my 15*F winter quilt into the realm of 0*F or colder. My plan is to layer the synthetic quilt on top of the down quilt and wear my down insulation pieces as needed inside the quilts. The synthetic quilt was sized large enough and is light enough to fully drape over my 15*F down quilt without any noticeable compression. In theory, having the synthetic quilt on top adds a little warmth and will also help with moisture management during wet winter trips by moving the dew point into the synthetic quilt rather than the down quilt. I haven't tested this yet in the field, but there were a few posts/articles over the last couple of years suggesting this approach works well for others.
FWIW, I've switched to quilts exclusively for the last 3 years or so. I don't even own a mummy bag anymore. They work a lot better for my needs and solve many of my issues with standard sleeping bags.