It looks like the title of a boxing poster: diffuser vs. humidifier. And the truth is that both are two heavyweights when it comes to treating the air in a given interior space.
It is natural that you confuse them because they both have many physical and even functional similarities (both seek to improve the air of a given private space), but they have quite different purposes.
Have you ever felt the need to regulate the humidity in your home? Or have you ever thought of scenting the air inside your room? So, you really need to understand the differences between humidifier and diffuser and only then choose the one that best suits your needs.