I have never looked at the Plasma, which is relatively new. I personally am a bit dubious about those vertical (as opposed to horizontal) baffles, which IMHO would potentially allow for more down migration. However, that's a very subjective opinion, probably also influenced by my age....

I hope you can find someone who has actually used this bag in a variety of conditions, preferably long-term, before you invest. Watch for reviews on Backpackgeartest.org and on various forums. Outside Magazine seems to like it, but I doubt they tested it on a thru-hike. It might be worth checking a bunch of 2012 online journals at postholer and trailjournals to see if anyone has this bag listed in their gear lists. (This might take quite a while as not everyone publishes a gear list!)

If you can get this bag from REI or backcountry.com, both of which have lifetime "satisfaction guaranteed" warranties, be sure to try out the bag on several preliminary trips to make sure that the down isn't going to migrate away from pressure points like shoulders, hips, knees. Actually, I suggest preliminary trial trips to test all your gear!

Again, the EN 13537 ratings are just about the same as for the Western Mountaineering 20* bags. The draft collar on the Plasma is, IMHO, a definite plus in below-freezing weather.

The Marmot Plasma 15 looks quite pricey. Unless you can get a really steep discount, it may cost more than the Western Mountaineering Ultralite--check prices first! If that's the case, I recommend the Ultralite, IF it's not too narrow for you (check your girth measurements while wearing your puffy insulated clothing).

My experience with quilts is limited to warm-weather use of my unzipped sleeping bag, so I really can't help you there. I personally prefer being enclosed snugly in a full sleeping bag--no drafts when I turn over, which I do frequently. This is definitely a "Your Mileage May Vary" situation. If you have a sleeping bag currently, try using it as a quilt (fully unzipped) on a cold night or two, perhaps with a blanket over it if it's not warm enough for winter weather conditions.