Hey @STAGfm, welcome to the Processing community!
The confusion is very common, even I faced the same while I started to learn programming.
In a sense, Processing will help you a lot if you want to build visual and interactive local applications where you want to import some Java library or work with Python. Processing would be helpful too if you want to perform serial communications like those with Arduino.
With Processing, you will be able to make your own .exe files which can run locally on your OS.
With p5.js, you will be able to make visual web applications which can run only on browsers.
But you should match your interests with what you intend to do with programming, and subsequently choose to learn any one of the two.
Tbh though, learning anyone of them will help. Both are very similar structure-wise. If you master one of them, the other one will look different only while reading the syntax.
I hope this helps and I could explain this properly.
You can refer to an article to clear any further confusion: https://happycoding.io/tutorials/p5js/which-processing