The Pro version adds the following exclusive features:
• Styled text format support - Bold, Italic, Underline, Color, Font, Size...
• Conditional message contents depending on optional tags value.
• 20 Custom Tags for advanced mail merge.
• Ability to process tags in subject.
• 13 date tags, Long Date, Short Date, Abbreviated Date, Long Time, Short Time...
• Quick selector in order to check/uncheck recipients depending on their values.
• Recipients creation directly from Filemaker. (Mac OS X only)
• Hyperlink manager to insert text and HTML hyperlinks tags with one click.
• Full Applescript support. (Mac OS X only)
• Full mySQL and postgreSQL database support.
• Secure SSL connection support.
I have added an entry to the FAQ for that topic:
http://www.maxprog.com/support/us/mbm/std_and_pro_differences.html