Any personal preference for Schmiding aside, all in all Zoo Salzburg is on a higher professional level (and I'm not the greatest fan of either).Regarding Austria, i would definitely say that Haus des Meeres is more Must See than Salzburg. You could also consider Schmiding more "Must see" than Salzburg, depends on what are you more interested in. Schmiding has far the nicer animal collection and some interesting exhibits. Therefore Salzburg has an unique landscape, but i think this is maybe not enough for Must see...