Both geese and ducks provide real down. Both fillings are natural, both insulate well, both keep working for years provided they are well cared for. The difference lies in the cluster size.
Goose down
Goose down has on average larger clusters than duck down. Larger clusters capture more air. Air is what keeps warm. The result: at the same warmth grade, a duvet with goose down weighs less than one with duck down. You feel it immediately when you pick up both.
Duck down
Duck down in turn has its own qualities. It is a touch more resilient, a touch firmer in the drape, and is generally more available, and so more affordable in price.
Which to choose
Which to choose? For those who love a light, almost weightless duvet, the choice falls on goose down. For those who prefer a touch firmer feel, or for whom price matters, duck down is an equally valid choice. Neither is better. There is what suits you.