Duck is gamier and greasier. Duck is like all dark meat, goose has a greater variety. It's got more grease than a turkey, but not so much as to make it anything but delicious. And a wild goose is different than one you get at the store, just like wild turkey is different, and wild duck is, too. The wild ones are generally a great deal more lean, require less cooking, and are smaller. I prefer wild every time. I'm going duck AND goose hunting this year in October and November, I hope to get several of each. I'll dress them myself and be able to use the organs, like the liver, to make extra good things.
But, to make it a whole lot easier just find a local poultry person and ask if they sell goose. They almost certainly will. I found a great place to buy them at the local farmers' market. That's always a good place to start.