Mike,
It depends on what you wish to do and how much additional features are necessary.
Outlook is my preferred email client for pop3 accounts and business use.
I use Windows Live Mail for hotmail type accounts(hotmail.com, live.com, and msn.com) where I desire synchronization of messages
and folders with the server. I discontinued use of Windows Mail except for use as an occasional newsreader.
Outlook is significantly more configurable and capable than Windows Mail(or its replacement Windows Live Mail), thus is is also
more complex and has a much higher learning curve.