The difference between down and feathers. The biggest difference lies in where you find them on the goose.
Feathers
Feathers sit on the wings and the outer layer of the body — protection against cold and moisture.
They are heavier and stiffer. The hard quill can prick and can break.
Down
Down sits between the feathers, on the belly of the goose. No quill. Lighter. Insulates excellently.
This is how a goose maintains its body temperature in cold regions.
Both above synthetic
Both are superior to a synthetic filling.
But down is lighter, softer and more durable. That is not an opinion — that is what one observes after decades.