jou is more common in “formal” writing (though there is also the very formal U that isn’t used as much these days TBH, and even another form Gij which is used by more religious Dutch speakers as a form of respect to a monk or nun), or when singing or writing poetry where it rhymes or scans better (like the Magdalena tune I found)
je is used for spoken Dutch and informal messages (like on a forum/texts). Even fairly formal environments with older people than me (like electronics / radio forums) now use jou or je than the more formal equivalents.
jou vs. je – WordReference Forums
