"Good people do not need laws to tell them to act responsibly, while bad people will find a way around the laws." -Plato
I'm torn a bit myself on this question. I also noticed that down provides a lot nicer "climate". Although it is very warm, I don't have the feeling to sweat a lot. With synthetic blankets, I often have the problem that I'm too cold or too hot. Down seems to work better over a much wider temperature range.
However, while I never really had problems with my down equipment getting wet, I noticed that the down in my sleeping bag lost loft and is clumping slightly after an estimated 1 year use (= adding the nights of use). Even after about half a year of nightly use I started noticing cold spots. I'm now about to send the bag off for professional cleaning, after which I expect it will be as good as new. But this would be a problem if I were to use the bag for longer without having the chance to have it cleaned.
I haven't used my under quilts as extensively as my sleeping bag, and I also think that the bag is a lot more stressed than the under quilts, since it's closer to the body and probably also absorbs a lot more evaporated water. But it's something to think about.
Depending on how long you will be hiking on the AT and how cold or warm you sleep, I would think about using a SPE and a pad under me, and a mix of a down and synthetic bag/quilt on top. The pad will come in handy if you are forced to go to ground, and you won't have problems with humidity. Personally, I like the feeling of a pad in the hammock. It's easier to keep warm, and it gives the hammock some structure. I also found that calf ridges are not an issue as much with a pad as with an under quilt. But I know that other people hate the feeling of a pad in the hammock.
With regards to the top quilt or sleeping bag: I have read quite a few posts from hikers who are using both. The down quilt or bag goes next to the body, and the synthetic quilt or bag goes over the down. You would have the best of both worlds: the synthetic material keeps condensation away from the down, and provides a safety net should your down get wet (although I'm not sure how warm you would be in a wet sythetic sleeping bag). The down will give you great warmth and provide a nicer climate. Here is a great post on this topic.
But all that said, mountaineers are still using down, as far as I know. And reading the books on the topic, wet down sleeping bags don't really seem to be a huge problem.
+1 on hutzelbein's suggestion to take a pad (and SPE if not a wide pad). There will be humid times where everything gets wet and stays wet. There are times of settling moisture that can affect the TQ at times, less so if you're using a bug net. Then there are times of ambient moisture that can affect the UQ. An UQ protector will help reduce that, but its not a necessity. Still, in either case, neither should get wet enough to worry about. However, in each case, if they are slightly damp, it might be a better idea to not put them in a dry bag in your pack, as that might hold that moisture in, more so than the pack would. Depends I guess on you keeping your pack dry, though.
But anyway, about the pad, there could be times where you might want to leave the UQ in the dry bag, and typically, those will be warm humid nights where a light pad will probably suffice. Something like this...
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First thing I would ask is how long are you planning on being out? You are section hiking right?
Others have given great advise, large tarps and weather shields work great. I have been section hiking for a little while now, I'm not as experienced as others, but, thousands of thru hikers have hiked with down without incident. Thats months of continuous use (or darn close too, how many zeros or hotels you stay in and not use your insulation)??????
Even PCT hikers use down through Oregon and north which see a tremendous amount of rain and moisture. And they don't have shelters to stay in like the AT has, they have to use what they have with them, so they are constantly having to set up in the elements. Yet they use down.
If you want to use synthetic's then by all means use it, many do and Arrowhead equipment makes some fantastic synthetic quilts. Also the advise of an SPE is solid, they work great, but the use of those is usually in place of a quilt not in conjunction with one although there are probably some who do??? But if weight and pack space (one or both) mean something then you are going to have to use down. One already mentioned the dri down (hydrophobic) down bring used by some cottage makers, and this is a very viable option, one that I am considering for my quilts (retro'd).
Like any equipment your insulation must be taken care of, you should never expect it to last without proper care, and that means laundering. Your sleeping bag or top quilt will not only get dirty from you and the elements but your body oils will get into the quilt and over time will affect the lofting of the down. Washing properly in a washing machine (following the directions) will help to keep your gear last, and will help it perform. Even if you are on a thru hike and don't have access to down wash, a plain water wash is better than nothing at all, make sure you use the proper machines though.
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Wow guys! I am blown away by all the good advice shared so generously. Thank you so much! I feel better prepared now, and will probably go with the down (maybe even the hydrophobic kind), and a large tarp.
If you're interested in the topic, I posted this same question in the top insulation forum, and got great answers there as well (not sure if this was the right thing to do).
Down. There are plenty of opportunities to pull off the trail and mechanically dry any real damp or wet.
I'm a big Syn fan in the proper situation. Anywhere you have the opportunity to get back to your vehicle or pull a zero, when in a wet way, gives the green light to down IMO.
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I'd probably wash a down under quilt in the washing machine, though. It's only a faction of the size of the sleeping bag.