Here is a summary of the comments:
- Plone is an enterprise-grade CMS that can support complicated workflow and permissioning and high traffic volumes. Drupal is targeted for smaller sites.
- Drupal's LAMP based architecture is more open than Plone's Zope (all in one application server and database).
- Python is higher performing and more object oriented, but more difficult to learn than PHP.
- Plone has fewer but more polished releases than Drupal
- Drupal's UI is very intuitive and requires little training