No matter what the insulation, you have to keep it dry! A soggy synthetic bag is no warmer than a soggy down bag (been there, done that, posted the story here several times, as those of you who've been around here longer know).

With a down bag you do have to be a bit more careful protecting the insulation from your body moisture and from condensation inside your shelter or bivy. Especially when it gets below freezing, your body moisture may condense or even freeze on the inside of the outer shell, which means the moisture will be in the down. That's where a vapor barrier comes in handy. Just don't wear anything more than a base layer inside the vapor barrier--your puffy jacket, etc., needs to be outside.

Re the Western Mountaineering Ultralite--I have one and love it! I especially love the draft collar, which I can snug up around my neck while leaving the hood a little more open for ventilation. However, the Ultralite is a pretty narrow bag. Check the girth measurements of the bag (on the WM website) against your own (measure shoulder girth over your arms and with your insulating jacket on). It may be that you'll need the wider version of the 20* bag, the Alpinelite. While you don't want a lot of dead air space to warm up inside the bag, you do want to be able to wear all your insulation inside the bag without compressing the down!