Google Docs are organized around individual authors. Wikis are organized around information and content.

This difference is most obvious when an individual leaves an organization.

Content authored by that user usually remains accessible in the Wiki (and the organization)...

...while Google Docs can be, and often are, deleted/offlined when their "owner" is de-provisioned.

Accidental organizational amnesia will increase as docs win institutional content wars on the strength of easier creation of info...