Don't know how I missed this post. So I'll make up for it now.
I made myself a dandy down quilt about a year and half ago and its definitely my go-to bag. I used 9 oz of 800 FP down which gives me around 2" of loft. I've used it from temperatures ranging in the upper 50's to mid-to-upper 20's. Its hard to give sleeping bags a rating since it depends heavily on things like your metabolism and what you wear to bed. I've used my 15* rated bag during the summer by unzipping it and using it as a quilt. You can also push your bag lower than its rating by wearing your insulating clothing or rain gear to bed. The time I used my quilt down into the 20's I was wearing upper and lower base layers, my hiking clothes, two pairs of socks, liner gloves, a fleece hat, and my rain gear. Basically all of the clothes I had with me.
To answer your temperature rating question more directly, most manufacturers fudge their ratings, just as they fudge the weights. If you want a bag that delivers to its rating, buy from a company like Western Mountaineering or Mont-Bell. Overall they tend to be the most honest in terms of temperature and weight. I take any stated specs from The North Face, Marmot, Mountain Hardware, etc with a grain of salt. Take a scale with you the next time you go gear shopping and compare the actual weight with the advertised one.
And for down vs synthetic, I still contest that neither will keep you very warm if it gets wet. Synthetic may take a little longer to wet out but when it does it won't keep you warm either. In the year and a half I've had my down quilt I have never had a problem with it getting wet. That includes last week, which I spent hiking in Colorado and got rained on for the first 3 days. Including an all night thunderstorm in which my tent decided to develop a leak right above my face. There was a little moisture on the foot end of my quilt in the morning but not enough that it saturated the shell or made the down collapse.
How late into the winter are you planning on camping? A 20* bag would probably be a good compromise if you plan on camping into the really cold months. What I've done with my quilt is pick something that is middle of the road and supplement it with extra clothing (which I already bring) if the temperature dips rather than having a bag for the absolute coldest temps I can imagine using it in and just dealing with the extra warmth (and weight) the other 80-90% of the time I use it.
I hope there's something in my ramblings that helps you make your decision