For what it's worth, as a user of the language I would like sets to behave as much as possible as-if they were basically dicts that map all elements to `()`.  That way I'd have to keep one less mental model in my head.

I deliberately say 'as-if' because when I'm a user of the language, I don't care how it's implemented.  (Just like I don't have to care as a user that we have (at least) two different ways dicts are represented internally.)