When I first got into web application development I tried both.  Plone was
more intimidating to me than Drupal for the following reasons:

1.  It was not based on a standard simple AMP Stack.  by AMP I mean Apache,
MySQL, PHP.
2.  I found the web site difficult to find tutorials and help, and the
forums for newbies was not user friendly at least for me.
3.  I was very new to high level scripting languages and PHP was and still
is used more for web applications than Python.
4.  By contrast not only did Drupal have easy to find tutorials for the
framework but it even has tutorials on learning PHP.
5.  There is a lot of information/documentation for modules.

This was my experience three years ago.  Now before anyone goes crazy on
me,  since this time I have come to really appreciate the elegance of
python code in comparison to PHP, but at the time I new very little about
it.  I believe that Plone is more extensible a framework than Drupal.  But
here's a question does Plone have API's or modules to connect/extend it's
capabilities with other applications like Alfresco, or Uberecart.  I know
Plone is very powerful and scalable to very large enterprise level
development, But then Drupal, combined with Alfresco, if
developed/implemented efficiently can replace, and even convert much of
traditional filesytem data such as docs and excel files and place the data
in enterprise class CMS.

Drupal is very flexible with style templating making it simple to
modify/customize a web application to a specific customer.  They even have
there own take on the CSS Zen Garden.  Because of this more web designers
are getting into it for ease of site design without coding requirements.
Drupal is also easily set up to run multiple sites through a single
install.  With one installation you can host hundreds of sites as a VSP
with minimal up keep.

I really should look into Plone again because I am sure it can do most if
not all of all I described above.  But of all of the communities I have
experienced (considering forums, websites, tutorials, wiki's, and number of
easy to access free modules) I like Drupal's over Joomla, Word Press, and
Plone.  To me for open source apps, the apps community is everything.


-- 
James