Just some notes.  Of course one could use both.

piwik (1.2.1):

• automatic update from web (though be careful)
• assign user roles per website
• unlimited conversion tracking / goals
• support forum
• some bugs and broken reports – visitor log broken

owa (1.4.1):

• no flash
• better navigation (though this is subjective)
• latest visits works correctly with timezone
• tracks domstreams and dom clicks
• action tracking – could be very useful for search tracking
• goal funnels
• some bugs and broken reports – top pages today broken

Further reading:

• Comparison « Open Web Analytics
• why should I go with Piwik over Open Web Analytics (OWA)?