WordPress is a delight if you need to get something up quick and easily, it's also fairly easy to extend.
Joomla, well, you couldn't pay me touch it nowadays. I once inherited a site for a department at a major university, which was powered by it (they now use a homegrown solution built using the Django framework). While there are tons of modules and components out there, quality is lacking. We had to roll a lot of our own stuff, even though components/modules for what we wanted existed. Development on its creaky foundation caused nothing but headaches. At times it really felt like, "what's going to break today?" If you're just wanting an easy to setup CMS for a fairly small site, then I guess Joomla isn't so bad; otherwise, I'd stay clear.
That said, from a development stand point, I'd choose neither and go with Drupal instead.