Hi Eli – The code base of the two is actually quite similar, and they have similar a look and feel.
The BP Docs plugin is designed to create a private wiki for groups. Only members of the group can contribute, and according to visibility settings, the docs can be viewed by certain members of a group, all members of a group, or everyone.
BP Docs Wiki creates a sitewide wiki that lets everyone who is a member of the site contribute and view. Depending upon how you set up you site, non-members can view the wiki as well. But only logged-in members can contribute.