Not sure if you read
http://www.zope.com/Training/Z4ITraining. At the moment OpenACS is clearly missing something like this, so this would be an advantage (training on how to use Zope for your intranet). OTOH, I'm sure if interest is out there that myself and a couple of others would be able to give you a good handson session :).
Furthermore, there is no OpenACS book out there (yet). You will have to wade through the documentation and use your printer.
There is no single driving entity behind OpenACS like it is behind Zope. This is good as long as the single entity stays in business.
You get to learn Python. From my very limited experience with it, it is harder to get than TCL. And therefore understanding the Zope API most likely is more challenging than getting the OpenACS API (though a couple of developers might want to disagree with me here).
Zope was traditionally a CMS. OpenACS has always been a community plattform. Therefore, if you want to use Collaboration within your intranet, your are most likely going to find the better solutions within OpenACS, due to the focus of the software. If you are looking for a publishing software for your intranet OTOH, Zope might be the better choice.
But to really help you you'd have to answer Dirk's questions and decide, whether you are going to play with the code yourself or expect more out of the box solutions.
