I think you meant Purescript vs Elm. disclaimer: I have used neither.
Elm has its own package manager, not sure if that is good or bad!
The biggest difference is that Elm is almost incidentally a similar strongly typed functional programming language and its biggest focus has always been on FRP (functional reactive programming) to simplify UI programming in the browser.
(I.e. what are the relative strengths and weaknesses of these two "modern languages targeting JS" offerings?)