Generally speaking, the best down comes from larger, more mature birds. When age and maturity are equal, goose down is better than duck down. However, down from an older duck is better than down from a younger goose. Aa high quality white goose down plumule has an extraordinarily high warmth-to-weight ratio. A duvet or sleeping bag filled with this down will be very light and incredibly warm. And it will last for decades.On the other hand, a poor quality white goose down will not loft up, and has a rather low warmth-to-weight ratio. Even if there is more of this down by weight, it won't be as warm. Down from younger birds not only tends to have poor filling power, it will also tend to collapse in a relatively short time, because its fibers are too fragile. This is usually the difference between an inexpensive goose down product, and an expensive one.