Signup user notification email VS multisite

I’d like to change both emails users can receive when registering.
The email is different whether the admin adds the user or the user registers himself. I found both functions and I did manage to change one of them.

Here is the code when admin adds a user, and is read only when on multisite:
if ( is_multisite() ) {
http://pastebin.com/V2URPfTt
}

And here is the code when user registers himself, followed by the code I first tried to change the content of the email:
http://pastebin.com/PwEYUuTj

I then tried this but it didn’t work:

if (is_multisite()){
    function my_theme_signup_user_notification_email(){
            return sprintf( 'Admin invites user');
    }
   add_filter('wpmu_signup_user_notification_email', 'my_theme_signup_user_notification_email', 11);
} else {
    function my_theme_signup_user_notification_email2(){
            return sprintf( 'User registers himself');
    }
   add_filter('wpmu_signup_user_notification_email', 'my_theme_signup_user_notification_email2', 11);
}

With this, no matter the way the user is registered, he always get the “Admin invites user” message.

Any ideas? thanks