The answer is...it depends.
Goose down is only superior when you compare two birds of the same age and maturity.
The lowdown on down
The quality of down directly relates to the maturity of the bird. Fibres from the down of an immature bird are not yet developed. Therefore they are scrawny and are fragile, they collapse easily and in a short period of time. What does that mean in terms of a duvet? The duvet will not have a high loft (pouffy factor) and will not be as warm, even if you increase the weight (amount of down "plumules" or clusters). When you have a duvet made of down from a mature bird, it will be lightweight, produce great loft and will be very warm.
The ultimate down
Eiderdown is "la crème de la crème" when it comes to down and interestingly enough comes from the Eider...duck! They can be found along the northern coasts of North America & Europe, as well as the Eastern coast of Siberia. What makes Eiderdown so special? The clusters actually cling to one another, resulting in ultimate insulation. Eiderdown is the most expensive down you can find. It is harvested from the nests by hand without disturbing the birds. Only 15-20 grams of down are allowed to be recovered from each nest.
A high quality duvet is an investment worth making. Take your time to find out exactly which type of down is best for your personal comfort preferences. Make sure that the cover is a good quality sheeting and tightly woven to make it down proof. If you choose a high quality fill (down) your duvet can last generations!
Choosing your down is the one instance in which it is okay to ask about age!