There is no clear answer for this question because it all depends what kind of project you need to create. It is good to know both CMS's because then you can fit solution for your/project requirements. Saying than one platform is better that the second one is usually related with lack of knowledge in some areas. For small projects, blogs, single pages for sure WordPress is much user friendly but if you need to create big portal, with user roles, managing different content etc., probably Joomla is a better choice.
If someone say that Joomla is better or WordPress is the best it may be true but only for particular purpose. From developer point of view the approach in this platforms is different but by different I can't say better. Some things may be easier in WP, some in J! but there are no remarkable limits in any platform.