Check out the series of articles of web app toolkits:
http://www.lerner.co.il/atf including Zope and OpenACS and others. The last "Choosing Tools" articles summarizes the differences.
Of course, which is "better" really depends on what you are planning on building. From my limited experience with Zope, I think OpenACS is a better platform for community and collaboration, and Zope is stronger in content managem,ent.