Software Design: Model method which model should belong

I am pretty sure this has been asked many times but I cannot think of a good search phrase to get some useful results, so I am going to ask again:

Lets say I have 3 model domains (apps), if expressed in django framework:

class User(Model):
    pass

class Content(Model):
    pass

class Comment(Model):
    created_by = ForeignKey(User)
    content = ForeignKey(Content)

If I want add a method to create a comment object, which Model should I implement the method in? I am in the dilemma between Content and User.

If it's user centric:

Class User(Model):
    def add_comment(content, message):
        ....

If it's content centric:

Class Comment(Model):
    def add_comment(user, message):
        ....

Is there a golden rule to decide? Or does it matter?