Hi Mike,
Please find as below.
Extended Notification - Sends email to user's outlook when ever he got a work item. Only configuration is needed, so less work. Also user has the option to take a decision from Outlook.
Automatic Forwarding - If user got any notification to SAP inbox[only notification not a work item], it will automatically forward that SAP notification into user's outlook.
SO16 - Same as Automatic forwarding except its a administrative T-code which is used to configure automatic forwarding for all users.
CL_BCS - Needs to write lot of codes to send an email. Here the flexibility is can add HTML codes to format the mail, can add own texts and can get the email from infotypes or from user master [The other three options only will get the email ID from user master tables] ..
Thanks,
Vijay.
