What are the main differences between Arnold and Renderman in Maya? Couldn't choose which one to learn.

Answer 1

Preference/ minor functionality differences.

Renderman is a proprietary plugin created by Pixar, with specific functions used in their workflow. Arnold was created by an artist named Marcos Fajardo

Arnold is more for artists who want to use a GUI. Renderman is more for serious programmers who want to totally customize the way things are rendered by the renderer