Technically, there's no difference between goose and duck down (the highest quality down in the world comes from the Eider duck).
The quality of the down depends on the age of the bird it was harvested from - mature down being what constitutes a higher fill power due to the larger size and density of the plumules. Otherwise, it simply comes down to the fill power(quality/maturity).
The only complaint I have seen regarding duck down is that it can have a slightly gamey odor if it wasn't processed/cleaned correctly. I haven't experienced this myself, and own a sleeping bag and a couple of jackets filled with it.
Experience is the worst teacher - it presents the exam first and the lesson later. - Unknown