Zope 3, far and away. There's great documentation, quite a few load handling options (different types of caching and ZEO to distribute ZODB caches to multiple machines). Zope 3 aggressively favors small cooperating objects (Zope 2 was inheritance heavy, making customization, extension, etc, a big chore). There are two books in print already and it's quite mature already.
Don't discard Zope 3 becaue of what you've heard about Zope 2. Zope 3 is a very different animal. -- http://mail.python.org/mailman/listinfo/python-list