In the fall I am going to Nepal for a trek. I need a new sleeping bag that goes down to 0 F. I have always used synthetic bags in the past. I know down is not good when it’s wet but I don’t think that is an issue here. I won’t be carrying my own gear so weight is not really an issue for this trip. Are there any advantages of down for this type of trip?
In my experience hiking and camping, down is far, far superior to synthetics when it comes to warmth. The few online sites I’ve checked for references seem to agree, down is superior to synthetic for warmth and weight. But if it gets even a little wet, it can lose a great deal of its insulative qualities.
ETA: Purely anecdotal but, ever since I bought my big down comforter and futon set, my apartment has been completely unheated during the winter (which can get pretty harsh here). So much warmer than the cotton/synthetic comforter I used before.
I’ve dumped all my down gear for synthetics. Yeah, down is better, but I want something that will work under the worst conditions. Because that’s always what you run into. 
This is a really good point. Unless you can absolutely guarantee yourself that you will not let your down gear get wet, then it is the best solution. However, that’s a pretty difficult guarantee.
With proper care and consideration, it is possible to keep down dry in otherwise wettish conditions, but it is quite difficult and, should you get a down sleeping bag, you should probably read a few online guides about down care. Especially if it is your first.
If you are trekking at high altitudes, like one might in Nepal, down is attractive because it is so lightweight and compressible compared to synthetic. Just be sure you won’t get caught unaware in a downpour. Wet down is absolutely miserable.
Go with down, and get a proper waterproof storage sack. If you’re tenting, make sure your tent won’t leak as well. A good down bag will have a water resistant outer shell that’ll handle some condensation, but if you get it wet nothing short of a dryer (home or laundromat) will dry it out. Tip: toss a couple tennis balls inside the bag when you’ve got to put it in a dryer - they help break up the clumps of down.
Eventually, it’ll need washed. NEVER have a down bag dry cleaned. Those chemicals will not come out. Submerge in a bathtub a small amount of very mild detergent and hand wash.
The choice between down and synthetic at 20 degrees is something to think about, but a synthetic zero degree bag is still significantly heavier and bulkier when you’re talking about hiking. If you’re car camping then it doesn’t matter.
It’s not that the chemicals (perchloroethylene/PERC) won’t “come out”, they come out just fine, but when the do they take all the residual oils off the feathers which prevents down from clumping and losing it’s loft.
That’s it. I mis-un-remembered that.
I trekked for 6 weeks in Nepal with a down bag and had no problems. Yes, down is a problem if it gets wet but it’s fairly easy to prevent that and not difficult to dry them on a trek like this since your days aren’t spent all day hiking. Now, I was carrying my pack so weight and size was important to me.
New synthetics are supposed to be better and lighter. I wonder if down being warmer is partially a sort of placebo - people expect it to be warmer.
Loft - warmth. And new synthetics are better (compress better, weigh less) then older ones. The trend is going to synthetics for a good reason, but it’s not quite there yet. For a trip like this, where I wasn’t carrying my own pack, I’d go with the synthetic.
Also be damn careful regarding the ratings, ie 0 degree bag, 20 degree bag…
Some manufacturers ratings are legit, if not conservative. Some are outright BS.
Do some heavy research and get a good bag. If you are cold and miserable and can’t sleep its gonna really impact your trip. Laying awake all night cold just waiting for the night to be over so you can warm up in the rising sun gets old fast. Like in the first 8 hours.
I am sure you can find some good impartial reviews on the internet.
I will likely buy a known brand like REI, Sierra Designs, North Face or Marmot. I think those companies rate their bags correctly. I have a 20 degree North Face that is nice but old and the rating is correct.
Bag rating are only useful comparing to others by the same company. All those companies use legit but different methods of rating their bags. And those numbers mean nothing until you calibrate your own sleeping comfort with theirs. Some people are cold sleepers and won’t feel comfortable in a 20 degree bag at 30 degrees. Others (like me) are warm sleepers and can take my 15 degree bag down to 0.
So, don’t take the rating on any bag as an absolute. If you’re a cold sleeper, go a good 10 degrees warmer then you think you need.