What differences are there between a work item and a notification mail?

a) The work item cannot be used to notify several users.
Mails can be routed to several users, just like work items. When a mail is sent, and one recipient reads and deletes the mail, all other recipients will still have access to their own copy in their own inbox. However, when a work item is processed by one of the recipients it will automatically disappear from all the other inboxes. So you can see that a work item is unsuitable for notifying several users.

It is also worth noting that a mail can be forwarded in many different ways (fax, internet...) whereas the work item cannot.

b) The work item holds up the workflow
When the workflow sends a mail (usually as a background step) it continues with the process immediately after transmitting the mail. When a work item is generated, the workflow will not continue until the work item has been processed. This slows down the process. Occasionally this is what is intended (using the work item as an approval step without the ability to reject) but usually you will better off using mails for notifications.
Note: You can send business objects as references with the mail either as a business object reference attached to the mail or as an URL (ABAP required).