I worked with joomla from some years and start wordpress few months ago.
Joomla is much better to develop than wordpress and the customization like modules in any page and position is very good. The biggest problem of this CMS, in my opinion, is the update, so the security. Yesterday I was trying to update joomla 2.5 to 3.4, following the oficial tutorial it said "cross the finger", so they don't even know if it will work? Of course that was a complete failure and the website had to stay in this old version. Even trying in another website updating 2.5.10 to 2.5.28 was enough to get all the website bugged and of course the security was really bad.
I tried wordpress and all my problems were solved. The plugins are much better than joomla. At the moment I guess Admin tool from akeeba backup is the best security for joomla, but next to plugin like wordfence and other from wordpress, admin tool is really small.
There is even a plugin to auto update wordpress, plugin and theme. The community is bigger and that make the big difference. About the customization well there are some plugins like visual composer or child-theme to help.