Actually, I just picked up an excellent book on Joomla to make you familiar with the basics:
http://www.amazon.com/Joomla-Users-Guid ... 014&sr=8-1
There's several others as well, but I happened to stumble upon this one, while I hadn't found any books on Joomla 1.5 in a high-street store yet.
Wordpress, by origin, is a blog tool, which is fine if your site is going to be a blog, mainly. I don't have any experience with it, but I would expect that makes the possibilities of extending it a little limited. Joomla is more like a website framework that comes with CMS features out of the box. But it doesn't stop there, you could get it to do just about anything you might want to do with a website, given there is an extension to do what you need. If you're a programmer yourself, there's very little you couldn't do.
If you are going to do any kind of serious research into this, you should definitely also include Drupal in your studies, it is like "the other" big open source CMS. Last time I checked, integration with phpBB seemed more complicated, but that might have changed.