It is hard to say what is better. Both tools have their own advantages. Ghost and WordPress are open-source tools. Both are flexible, reliable, and secure. And both you can use to create a convenient personal or commercial blog.
Now let us have a look at the differences. WordPress is written in PHP and Ghost in Node.js. Ghost is more focused on blogging while WordPress allows you to create very different websites. Ghost has native tools for SEO and WordPress needs additional plugins. WordPress is more popular and has a bigger number of themes. But Ghost is faster than WordPress and supports Markdown. Thus, if you are looking for a blogging service, Ghost will be better because it is created for blogging. But if you need a fully-functional website, you might try WordPress.