It would work applied to either. The beauty of framework builder is the way it allows you to organize pages and includes however you want or need, so there is no correct place. Where you add it depends on the overall application.
If the email is inctricately tied to the form, then it should be on the page with the form.
However you may have a dynamic plugin with multiple forms and they all send an email. In that case you would be able to apply the email to the original page and it would be able tow work with any of the forms.