Conventional wisdom claims that goose down is better than duck down. And that is true. It’s possible to achieve a higher fill power with goose down than duck down. But when the fill power is the same, the quality is the same.
Fill power is the number of cubic inches occupied by 1 ounce of down. Down with higher fill power is of higher quality; down with lower fill power is of lower quality. Or to say it another, more simple way: high fill power down is puffier and better.
There are a couple main things that make down puffy. First, the ratio of down to feather in the mixture has a big impact on how high the down lofts. Feathers do not loft, and have no fill power. If there are a lot of feathers in the mix, then it prevents the down from lofting as fully as possible. Down / feather mixtures range from 50% down / 50% feather all the way up to 95% down / 5% feather. Less feathers in the mix, allow the down to loft higher.
The second factor, determining loft ability, is the size of the down cluster. Larger down clusters are generally stronger and able to loft more fully. This is where goose down has the advantage over duck down. Goose down has larger clusters, and is able to achieve a higher fill power rating. That means more warmth with less weight in your down comforter.
So goose down comforters are capable of a higher warmth to weight ratio, but in practice this is not always the case. For example the Williams Loft Classic Duck Down Comforter uses a duck down fill. It has a 690 fill power and uses a 92% down mixture. But there are many goose down comforters on the market with with only a 550 fill power and 75% down mixture. In this case the duck down comforter offers a superior warmth to weight ratio.
Simply put, the down comforter with the highest fill power is the best, regardless of whether it’s filled with goose or duck down. However comparing down comforter fill power from different companies may give an apples to oranges comparison. Quality varies widely and may not be as stated on the packaging.
Only companies with stringent quality control procedures in place are able to verify their claims on the product labeling. Good down bedding manufacturers verify their quality with 3rd party services like the Downmark Certification or testing by the International Down and Feather Laboratory.