The major difference is fallback behaviour. If a client (web browser or
email client or similar) receives a file with a text/* type for which it
has no special handler, in the absence of other context it is expected
to treat it like text/plain, and show the file to the user as text. If a
client receives a file with an unknown application/* type, it is expected
to treat it like application/octet-stream, assume that viewing the file as
text would be pointless because the user wouldn't necessarily understand
it anyway, and offer to save it or open it in an external program instead.
For scripting languages like sh and Python, I'm not sure: either way
could be appropriate. Which is more common: sharing scripts as source
code to read and edit, or sharing scripts as executables to download
and run as-is? If the former, text/ makes sense, if the latter,
application/.