The correct use of em and en
The em dash (— U+2014) is used to indicate a sudden break in thought ("I was thinking about writing a--what time did you say the
movie started?"), a parenthetical statement that deserves more
attention than parentheses indicate, or instead of a colon or
semicolon to link clauses. It is also used to indicate an open range,
such as from a given date with no end yet (as in "Peter Sheerin
[1969--] authored this document."), or vague dates (as a stand-in for
the last two digits of a four-digit year).
Two adjacent em dashes (a 2-em dash) are used to indicate missing letters in a word ("I just don't f----ing care about").
Three adjacent em dashes (a 3-em dash) are used to substitute for the author's name when a repeated series of works are presented in a bibliography, as well as to indicate an entire missing word in the text.
The en dash (– U+2013) is used to indicate a range of just about anything with numbers, including dates, numbers, game scores,
and pages in any sort of document. It is also used instead of the word
"to" or a hyphen to indicate a connection between things, including
geographic references (like the Mason–Dixon Line) and routes (such as
the New York–Boston commuter train).
It is used to hyphenate compounds of compounds, where at least one pair is already hyphenated. The Chicago Manual of style also states that it should be used "Where one of the components of a compound adjective contains more than one word," instead of a hyphen. Both of these rules are for clarity in indicating exactly what is being modified by the compound.
Conclusion : Use dash