Since you're in Colorado, your normal backpacking environment is harsher than that of many summer backpackers. At high altitudes in the Rockies, you can expect below-freezing temperatures any night during the summer. It can snow any time, too. Last summer in Wyoming's Wind Rivers there was a foot of snow in the alpine areas on Aug. 8 (just before I started) and again on Aug. 15. If my dog hadn't gotten sick, forcing me to cut short my trip, I'd have been up at 11,000 feet for the Aug. 15 storm (it would have been a good test of my gear!). Even around home (Pacific NW), I like to backpack in shoulder season in which similar conditions exist by September.
I therefore use a 20*F bag (Western Mountaineering Ultralite) for summer backpacking. Admittedly, it's too warm for warm nights in the Oregon Cascades, but I can leave the full-length zipper open or even start out on top of the bag when I go to bed. Even during a hot spell, nights at higher altitude cool down enough that I usually have the zipper mostly closed by morning. I want a sleep system that, with supplemental insulating clothing, will keep me warm (or at least not shivering) to the low teens (F) if necessary, because I have regularly encountered such conditions at high altitudes during my summer trips.
A well-insulated pad is really important in those conditions. I found out the hard way last fall that the NeoAir is not designed for below freezing conditions unless you supplement it with a nice thick CCF pad (a GG Thinlite pad definitely did not do the job). Maybe it was just as well I did miss that Wind Rivers snowstorm! I realize that some here have survived the NeoAir with a thin supplemental pad in below-freezing temps, but I'm a cold sleeper! Do try out any pad you buy at home on the floor for a couple of nights; comfort is also important. Those of us who are older need more padding underneath, as do side-sleepers. Sleeping styles vary considerably by individuals, so you need to find what is comfortable for YOU.
I personally like to have my WM bag (with its wonderful draft collar, worth at least another 5* of temperature rating, IMHO!) snuggled around me when it's cold. As others here have suggested, I'd start with a sleeping bag, with a full-length zipper. The full-length zip will let you use the bag as a quilt (I've done that for warmer nights). If you decide after several cold-weather trials that you prefer the quilt configuration even on really cold nights, you can sell the bag (used but well-cared-for high-quality down bags are easy to sell!) and get a quilt.
The high-end bags are generally fairly accurately rated. Western Mountaineering bags are actually rather conservatively rated, considering that as a really cold sleeper I can be comfortable well below its rated temperature!
Be sure to pay close attention to girth measurements. Measure around the outside your shoulders wearing several insulation layers (puffy if possible). You don't want to compress either the bag's insulation or your clothing's insulation, which you'll do if the fit is too snug! If you get claustrophobic in a snug-fitting bag, get something wider. If you look at the Western Mountaineering bags, you'll see that they have narrow and wide bags for almost every temperature rating.
Oh, and if you're worried about moral reasons, most countries have strict laws against plucking live geese. I have read that China is not one of those. In Europe, goose meat is a popular item on the menu, so you won't find geese being killed only for their down, either. A few companies only use the down that live geese shed as part of the normal moulting process; of course those are the most expensive!
Having been there and done that, I can tell you that a wet synthetic bag is every bit as cold as a wet down bag! (That was the same trip during which I learned about being extra-careful not to select a tent site that could turn into a lake during an all-night cloudburst followed by several inches of snow!) Whatever your insulation, it's absolutely vital to keep it dry! Keep it inside something that will keep it dry if you slip and fall while fording a stream, whether a waterproof pack liner or a dry bag. Always unpack or pack it under your shelter. If you get what we Northwesterners call a "sunbreak" during the day, stop and air out your sleeping bag.
