What are the best conditions to use a synthetic sleeping bag vs a down one? Can you always use a down sleeping bag as long as you keep it in a dry waterproof bag?
The answer is very dependent on the prevailing weather conditions where you are active, and what your budget is
Given that I have used both styles, and love them both in different conditions I would say make your decision after considering the above and speaking with local experts who have used the product in your climate area. Knowledge from a local makes all the difference.
This appears to be a slight misunderstanding. The problem is not keeping the bag dry when transporting and storing it. The problem is mostly condensated moisture and water vapour from your body.
For example when you sleep in a tent in the winter, there’s a lot of condensation on the tent walls. Some of it drops down and if you’re not careful, your bag could be wet after a few nights unless you can dry it somewhere. With bivy sack the situation can be similar. This can be solved with a membrane down bag that’s got a waterproof fabric on the outer side that will not let any water in.
And then there’s moisture from your body. For example if you stick your head into the sleeping bag and exhale into it, that’s quite a lot of moisture that will again condense somewhere in your down insulation and will build up day after day until you can dry the bag.
These are issues that many people never encounter, not sleeping outside in the winter, knowing how to work around them or staying out just shortly so that the moisture does not have the chance to build up that much. Which is why I agree with Dangeranger in that you have to know what you are going to do with your sleeping bag.
I’ve had about three synthetic sleeping bags, the last one very fluffy and warm. But when I recently bought a down sleeping bag for my girlfriend, there’s no coming back, my next bag is going to be down. It’s much smaller, warmer and lighter, and I haven’t run into any issues with it yet.
Goose-down bags are lighter and more durable as the insulation holds up MUCH better to compression than synthetic.
If you are going on long treks and really need to watch the weight, then a goose is probably better suited. However, keep it dry! Keeping it in a waterproof bag (as you suggested) is a must.
If you don't really go camping much (once or twice a year at best), I wouldn't spend the money on a goose.
The goose-down really has two major downfalls: