Help picking a quilt

I've never noticed any loss of loft (visually or thermally) in 2-3 days of humid/wet conditions. I live in the PNW as well (Whistler, BC) and I've done several trips with 4+ days of straight rain (ie. Vancouver Island) and I never observed any trouble, so my opinion would strongly be for down. My body heat seems to keep the humidity transferring out of the quilt but I've never gotten too science geek about it. I've done 8 day trips with rain roughly half the time and I never hit loft problems even though I never aired it out to dry. Winter is more of a concern for me, where the freezing point for moisture could be inside the quilt and then it would collect there. This only happens in true winter conditions though, so you probably wouldn't be using the same quilt anyways (and if you were there's steps you could take, ie. VBL).

For an insulating jacket that you'll be wearing outside the tent, synthetic might be the way to go, but for a quilt I'd definitely go down.

Personally, I think if you're in wet weather and you only want to buy one quilt, I think synthetic is the way to go. The weight penalty is getting less and less with better synthtic options on the market.

I did the CT last year with a down quilt, which is an amazing piece of gear. However, in huge rain and hail storms, my tent did not keep the weather out an I ended up with significant water inside my tent. it was a real effort to keep the down quilt from becoming saturated. If the weather had lasted with decreasing temps, it would have been a bad scene. After that I bought a synthetic quilt. I have kept the down quilt, but will only use it in places where the weather is more dry.

"However, in huge rain and hail storms, my tent did not keep the weather out an I ended up with significant water inside my tent. it was a real effort to keep the down quilt from becoming saturated".

I think the Op is more wondering about humidity and body moisture.
A leaking tent is a whole other ball game. = BAD
I wouldn't think it's usual to plan for a leaking tent, after all if we did we'd still be lugging 40lbs and no longer UL'ers.

My votes for a down bag/quilt.
We get pretty high humidity in Australia and I've never had an issue even in the rain for 4 days straight.
I did read somewhere that if your concerned about moisture and loft loss just get a little overfill to compensate. That's what I have done.

A few years ago my hiking buddy and I went out in temps just above freezing, with frequent rain and high humidity. We used hammocks with good tarps, and down quilts. Both of us saw significant reductions in the loft of our quilts, although the quilts still retained enough to get us through without being cold. With thinner quilts, we could have had real problems.

My personal recommendation is to use a synthetic quilt when you expect to encounter prolonged humidity.

I always take a warmer bag than needed by about 15-20 degrees so that I can keep a zipper open and let it breathe every once and a while.

Here are my thoughts on synthetics:

Too heavy but… they can keep you warm when wet.

For down:

very light but… they loose their rating when damp.

That being said, if you are careful with your down bag selection (go overboard in the rating by 15 degrees or so) and you just lose some loft you are still warm. If it gets soaked too much to any good at all then a synthetic would not help you out much there either. Best thing to do is make sure you are using a tent that is very well ventilated and pitched correctly to avoid rain getting inside… doing so will keep your down bag dry and be able to withstand a few days of rainstorms with no problems at all. To mee, the extra pound for a synthetic is not worth it (I own one and now it just collects dust in my attic).

I'm also going to throw a vote in for down. If your bag has a decent DWR coating, that will go a long ways in preventing spray and splash from soaking the bag.

Condensation is another issue, but like the others said, a well-ventilated tent should take care of that problem.

Synthetic is just a tad too heavy for my tastes, and it just doesn't pack down as well.