From version 1.8.8 on, you can create content diffs with PSN. Diffs can show you the changes in title and content of a post on update.
These are the available placeholders for diffs:
As their names imply, the first one contains the title changes and the second one the content changes.
Diff placeholders contain HTML tables, so they have to be used with HTML mails to be readable.
The placeholders contain the WordPress internal HTML diff format, which is used in the post revision admin. So the placeholders only make sense when used in HTML mails. Otherwise you would see the HTML source code in a text mail.