I like the idea of a down bag, too. But I can聮t help but think the humidity will pretty well kill it. So I聮d go with synthetic. It聮s not as if you聮re going to save a lot of weight by going with down, anyway聴not in a bag rated around 40 degrees.
Mountain Hardwear UltraLamina 45 Sleeping Bag
UltraLamina 45 Sleeping BagYou could, for instance, get a UltraLamina 45-degree bag ($170). The name is the temperature rating; it聮s strictly a warm-weather bag, but likely would be just the ticket for you. Its fill is a proprietary polyester material, which is a good thing because polyester is reluctant to absorb much water so will stay dry. It weighs a pound and a half, so it won聮t make a big dent in your luggage allotment. The Yachiyo 45 ($109) is very similar in weight and temperature rating, and it聮s also considerably cheaper聴perhaps due to fill that isn聮t quite as durable. But it would work well for you.
You can add some warmth to either bag, or have a standalone sleep-shell for very warm nights, with a Ripstop Silk Bag Liner ($65). I聮d recommend one in any event, as using an easily washed liner will help keep your main bag cleaner and extend its life.
And, for a warmer bag, look to 聮s Nooksack +35 UL ($170), a bag that uses Primaloft insulation, a material I like a lot because it聮s soft and down-like, yet resists moisture extremely well. It weighs just 28 ounces聴not bad!
So there you go. Hope you have a great trip!
The 2008 Summer 国产吃瓜黑料 Buyer聮s Guide is now on newsstands. Look for it online soon.