For short trips, overnight, weekends and probably up to a week or so, or any length of section or thru where you have the ability to pull off the trail and mechanically dry insulation, pretty much always down.

Anything longer, Bug-out, extreme distances from vehicle or pick-up points, or expected wet, severe or potential life threatening conditions, High Quality Syn.

Warmth, weight, and compression, Down is always worth the extra money.

High Quality Cottage Syn, will only save you some on the actual fill price. Craftsmanship for the shell costs are about or should be the same.

Compression and weight penalties, IMO should be offset by application and need rather than initial price savings or cost.

Combining the use of Down TQs and Syn UQs can probably help you make peace with both pack size penalties, and over all weight, when packing for short term damp or wet weather fears.