They're two completely different programs, one is a content management system and the other is a what you see is what you get (WYSIWYG) / code highlighting editor; so this comparision is void before we've already got started. You can't compare them.
To be honest, I dislike Joomla!. It tries to do too much in my opinion, and is a little bloated for my purposes, but that doesn't mean it's a bad application or it won't work for others - I'd just prefer to write something that's a little more suited to the project, that's all. It has got a large array of features to make keeping a site up to date a lot easier and faster.
And Dreamweaver is an amazing program, but again other people may have a different preference. It makes mixing PHP and HTML a lot easier because of the way it is capable of highlighting both simultaneously in the same file. Its default layouts are a little bloated, but it's easy to make your own. It offers helpful information about different elements in HTML and CSS and functions in PHP. This saves a great deal of time; it's a great program.
But as I said above, you can't compare two programs that do completely things, it's like trying to compare an oven and a fridge really - it can't be done.
-Luke.