Which one you use is entirely up to you -- however, here are some of the differences.
Most notably, MSN Messenger 6 has support for display pictures, custom emoticons, some new
file transfer capabilities (can send files through the server), a new(er) ' *** ' feature,
built-in games, and the custom backgrounds.
See the 'Learn More' links here:
Which one you use is up to you, and a matter of personal choice (and based upon your needs
and likes), however one thing to keep in mind, Windows Messenger must still be installed on
Windows XP in order for MSN Messenger to retain full functionality (Remote Assistance,
Whiteboard, Application Sharing, etc.).
Windows MVP, Messenger
Messenger Resources - http://www.yqcomputer.com/