You need to install a specific Processing app

p5 can run in any Firefox or Chrome browser just by visiting a website in any computer

Processing can easily communicate with Arduino and other software and hardware

p5 is restricted to things that can run inside a browser. While there are setups to, e.g., communicate with Arduino; they can be complex and error-prone.

Processing has a large number of different libraries available (pre-made code to do complex tasks without having to think about them yourself)

p5 has fewer libraries available

Processing has been around for more time so there are more tutorials and code examples available

p5 is younger and there's fewer available learning material

Processing uses Java, which can be a bit more complicated to write and reason about

p5 uses JavaScript, which is slightly easier to write and reason about

Processing needs to compile your code every time, which reduces iteration speed (the time between making a code change and seeing its result) to multiple seconds.

p5 is not compiled, so the iteration speed is less than a second

Processing code runs faster (the difference for our uses is almost none, though)

p5 code runs slower (the difference for our uses is almost none, though)