Zope 2 and Zope 3 are related, but different. Add-on products for one do not work for the other. The divide is shrinking - Zope 2.10 actually ships with Zope 3 included and increasingly makes use of that newer code.
Zope 2:
- is mature
- is solid
- has the largest community
- has a wealth of ready-to-run add-on [products]?
- is documented at http://wiki.zope.org/zope2 and http://zope.org
Zope 3:
- is a ground-up rewrite embodying the lessons learned from zope 2
- is solid
- is clean and robust, with much better separation of concerns for developers
- is documented at http://wiki.zope.org/zope3
General recommendation: as of 2007,
- site builders who want to get sites up and running quickly, with a lot of ready-made add-ons to choose from, should choose Zope 2, but keep an eye on Zope 3 and try to use its practices where possible.
- python developers who want to hook their existing python apps to the web in a clean way, with no pressing need for the Zope 2 add-ons, should use Zope 3.
See also Lennart Regebro's Zope2 vs Zope3 FAQ.