Both of them are irreplaceable.
E: There’s a joke stating that the letter C can be replaced K and S (but “ch” can’t be replaced by “kh” or “sh”). In a similar fashion, em dashes (commonly used to add relevent information like parentheses or to introduce a definition) has some of its usage overlapped with parentheses and colons, but still has some unique usages that can’t be replaced.
S, the harder to write character should be the one to go, this also adds compatibility with Cyrillic.
Sh should then be the one replaced.