The choice depends on your application...

DUCK: Ducks, being smaller birds, produce smaller down clusters and feathers. The blends produced from duck are a cost effective choice for applications like furniture, throw pillows, and other uses where insulation is not a big factor.

GOOSE: Geese, being larger birds, produce larger down clusters and feathers than those of similarly raised ducks. The larger clusters of down produced from geese make for a higher insulation factor. To that point, goose blends are usually the choice for clothing and bedding - where the higher insulation factor is of primary importance.