OK, so I'm slightly biased in that I run a company that does predominantly Plone development, but one of our biggest clients uses Confluence to actually project manage the Plone project we are developing for them. So in effect we use both, for different sides of the same coin.
Confluence is quite powerful, and some of the tools for previewing MS Office documents in your browser are pretty good. However where Plone shines is its flexibility. We use Plone ourselves for our own intranet and the great thing is it goes well beyond just storing documents:
* It shows a list of all latest SVN commits from our repository
* There is a shared calendar with SMS alerts, iCal integration, etc
* It integrates with our time tracking software to produce time tracking reports for each project
* There is a wiki on there for ad-hoc knowledge bits
* There is a directory of all our contacts with click-to-dial integration with our desk phones
* All our quotes are 'written' in Plone and converted to PDF to send to the client. It handles all the formatting via Apache FOP
* There are image galleries for both social company photos, and also library of screenshots for quotes, etc.
* Management dashboard with graphs of time spent in the past week on which projects
So whilst Confluence is pretty good, I think you'd be hard pressed to customise it quite to the extent where it really starts to deliver business value by integrating with your actual business processes and other software.