Just to weigh in publicly and speak more specifically to your caveat:
Since Processing is a standalone program, I assume it doesn’t require additional libraries beyond an OSC library to get SPi communicating with it. p5.js is meant to work within a browser, so you need to install node.js, do some command line stuff to set it all up and run it in the browser. (point - Processing)
With that said, you would need to install Processing on every computer you intend to use it on, whereas p5.js has a web editor (https://editor.p5js.org/) that is easily accessible with an internet connection. This would also allow students to access their work outside of the classroom if they wanted as well as share projects that they have made by simply sending a link to someone… (point - p5.js)
I think the choice really depends on your classroom setup, the amount of time you have and how receptive your students are to the initial setup process prior to actual creating something.