I use only down for all my trips. My topquilt is made with a Pertex outer shell that really sheds water. When I ground camp under a tarp I have a 7 oz bivy made or Pertex on the top and silnylon on the bottom. That adds one more layer to keep moisture at bay.
I've endured many nights of fantastic storms and never had my down wet out.
The only issue with down, in my opinion, is that in very humid conditions it will absorb more moisture than man made filaments. When we humans sleep we give off moisture in the form of perspiration. In the more humid nights, less of that moisture can escape through the bag. But, to me this is not a problem if the trips are only 1-2 weeks long. The bag is not going to wet out.
Also, on days when the sun is shining, it is easy to air the bag and get the little bit of moisture out.
Even man made filament bags have this same issue, but it is not as pronounced.
The only time down becomes a problem is when we are going to be on very long trips and enduring lots of rain, humidity, etc etc etc and no sunshine to air the bag out. Or, when we go into arctic regions in subzero weather for weeks on end without wear VBL to bed. (Read about Will Steger's dogsled trip to the North Pole.)
Down is by far the best choice for most of our trips. It will far outlast the man made filament bags by several years. Shucks, I still have a North Face Blue Kazoo that is older than dirt. Still going strong. A man made filament bag of the same age would be thin as roadkill.
Sorry if this sounds preachy...sure didn't mean to do that.
Buy yourself a good quality down bag and it'll be your friend for many years to come.