Geese and ducks both produce real down. Both are natural, both can insulate well, and both have their place. Still there is a difference that you feel.
Goose down
Goose down has on average larger clusters. Larger clusters hold more air. So less filling is needed for the same warmth and the duvet stays lighter.
Duck down
Duck down usually has smaller clusters and asks more filling for the same insulation. So it can feel fuller or heavier, without necessarily being warmer.
Origin
Origin matters too. Down from cold, northern regions often has better insulating properties than down from warmer regions.
Our choice
We mostly work with goose down because it gives much warmth at little weight. At the same time, the right choice always stays an interplay of quality, origin, finishing and personal feel.
No simple winner
Goose down and duck down are no opposites. They each have their own feel.
Why experience counts
Numbers and origin help, but it remains useful to feel the filling. Lift two duvets and notice weight, suppleness and volume. Then the difference becomes clearer by itself.