I'd like to respond to your questions directly - my expertise is in Joomla but I have been a co-designer on a Drupal project and have done a co-presentation with a Drupal designer regarding comparing the two:
* the site will expand over time from 50 pages to possibly 500 after several years
Both handle this equally well.
* The site has to be SEO friendly
I don't know specifics of Drupal, but the default Joomla install is not particularly SEO friendly, but that is easily with several extensions. I recommend sh404sef, which has a small fee.
* Is JOOMLA equal in SEO rankings or is DRUPAL better to get a better rank for a website?
I think that has more to do with the experience and knowledge of the person configuring the site's title's, tags, pages, etc.
* Is JOOMLA as safe as DRUPAL in regard to SECURITY and HACKING
Joomla requires a bit of configuration, someone who knows what they are doing and/or can read the excellent materials at Joomla.org. And a good secure webhost.
Latest whitehouse.gov is on Drupal. Not saying that Drupal doesn't require some smart people to run safely, just that somebody there thought it was good enough.
It will have about 3 categories which will include articles that will be added per monthly basis.
Both would handle this equally well.
It will also include BLOG page with post and no comments.
In Joomla, this would work by default, especially if you also don't want pings, trackbacks, etc. If so, there are 3rd party blog extensions.
* A Directory feature of approx 150 Contact Details for Businesses and their website links
In Joomla, take a look at MTree. Fee- based extension, but it kicks. We run a rather robest directory site with it and we're constantly amazed by it.
Hope this helps!
