This is up to your own preference but we recommend generating a new email in FYI.
You can create the email as "Draft in FYI" so that you can then change the filing details, set workflow, delegate tasks, adjust/add attachments etc. in the drawer before you send it.
We recommend using "Draft in Outlook" when you need to access functionality not available in FYI, such as sending to contacts from your contact list in Outlook, setting read receipts, delayed send, etc.