Atom was developed to fix some of the flaws in RSS. It aims to provide a cleaner, more consistent, and extensible standard for web feed syndication. Furthermore, Atom supports adding metadata and other custom features to your feed content, which is something that RSS lacks.
That said, RSS feeds are more widely supported by different feed readers and CMSs and have a larger set of tools and resources. On the other hand, Atom’s ecosystem is still growing, so its compatibility may not be as extensive as RSS.
With WordPress, you don’t have to worry about choosing one or the other because both feeds are already generated for you by default.