Well, considering they are both in the Ground egg group, they should be considered more mammalian than bird-like. To my knowledge they are supposed to be Duck-billed Platypus'. You see, that right there is where the confusion starts. If they were ducks, Nintendo probably would have made them part flying-type, considering ducks can fly...Still, as much as i hate to do this, i would consider them more like Platypus' than Ducks.
Than again, if i remember correctly, Platypus' are supposed to have poisonous spurs on their hind leg(s), but Golduck and Psyduck don't have any visible spurs...
I'm kind of Neutral on this one. I think Nintendo/Game Freak intended to make Golduck and Psyduck have features of both animals.