There is no doubt, Joomla and WordPress are very popular web platforms. Both are used for a wide variety of websites. Both have great support and huge collection of extensions and templates. Joomla and WordPress have a large community and numerous supporters. But there are some differences.
WordPress is a blogging platform. It is optimized for creating a blog and adding posts into categories. Joomla is a universal content management system (CMS). This means that it is not specialized for any purpose. Of course, you can create blog with Joomla, and you can also create almost any website with WordPress. The difference is how easy it is to adapt your system for a specific task. It is like creating spreadsheets in Word or writing documents in Excel. Both is possible but Excel is dedicated for spreadsheets and Word for document writing.
WordPress is said to be simple for beginners, but also Joomla doesn't look complicated either. With every new system you need to know the basics and get used to the user interface. Everything is simple after you get some experience and knowledge. Joomla and WordPress can be used as the first CMS to work with. There are many online tutorials and great support on the web. This is the beauty of well supported systems. When you encounter a problem you simply google for it.
Both systems have many thousand extensions. But since WordPress is more oriented toward blogs this is also reflected at the extension. Since Joomla is a universal CMS there are extensions available for every imaginable feature. This is where you can start understanding what universality is.
Joomla is more powerful CMS than WordPress. It is used with many companies as primary web CMS. As long as you keep up with the updates the security is not a problem. WordPress is rarely used for main website of a company.
Both systems have a huge collection of templates. You select the template that reflects the nature of your website. While for some you need to pay some small fee, there are many freely available templates. The difference between WordPress and Joomla is reflected also at templates. Joomla template is much more sophisticated and gives more power to the designer.
The conclusion is that both systems work well and the decision is up to you. If you have time then you should test both systems and select one that works best for your needs. If your only goal is a blog then you will probably select WordPress. But for any other type of website it is a better decision to choose Joomla.