Upgrading Joomla / Wordpress
Joomla will release upgrades regularly. They have been doing so for several years now. That’s o.k. when they stay on the same version like 2.5. or 3.0.
But as soon as they go a version higher you have to upgrade your Joomla but also your components. Usually I rebuild a website entirely when Joomla releases a new version (not an upgrade of that same version). When I rebuild the website I’m sure all components, modules and plugins will be compatible with my new version and will not cause any problems.
Wordpress does send out upgrades just like Joomla which you can press so that it automatically updates your installation. Wordpress recommends you deactivate your plugins before you upgrade to not get any compatibility problems.
To me it does look like it will be easier to upgrade Wordpress than it does to upgrade a Joomla website (without any compatibility problems). It might not require a complete rebuild of the website. I do think you can use the Wordpress themes for a long time, and on Joomla your themes are made for a certain version of Joomla.
In Joomla you can see when your Joomla version needs to be updated and press a button to do so. In Wordpress you can see that your Wordpress version needs to be updated but you can also see when plugins or Wordpress have received an update and you can update them just by clicking the button to update them. I think the update process goes easier in Wordpress than it does in Joomla, which is a big Plus for Wordpress users.