Rhetorical characteristics for informal writing would be in my opinion something more personal based on persuading, it’s a type of writing style that really connects with the reader on a more personal basis. A good example of this is rap songs in the current generation. I personally am not a fan of this style of writing myself.

Rhetorical characteristics for formal writing tends to be more professional writings, types of writing that even if it were about a personal topic, would be very filtered and professional toward the reader. A good example of this would be poetry or a research paper of some kind. This tends to be my preferred form of writing when it comes to writing; it feels like a better for of writing for me all in all.

Picture from “Corporate Citizen” http://corporatecitizen.in/issue15/debate-dress-code-formal-or-informal.html