from how I see it, the only difference is that the possibility of a duplicate mythical exists, whereas for legendaries it doesn't. we never see duplicate mythicals standing side-by-side though, and they're always presented as a legendary. e.g.
>having one Arceus makes sense, it's a legendary.
>having multiple Mews makes sense, it's a mythical,
otherwise there's no difference. since we never see the duplicates and they're presented the same, they should be considered the same.