Owncloud is for decoupling data from web applications, and retain "ownership". It allows users to choose where to store their social data independently of the web service they use. Owncloud compares to Dropbox.

Buddycloud is a social networking platform based on XMPP, and integrates with websites. BuddyCloud (remotely) compares to Facebook.

Also, Owncloud is 2 letters closer to the cloud.