Arduino is for quick validation and prototyping. Pick up an Arduino nano for couple of $ and try it out. The programming language is easy to pick up and will get you a quick app(though basic) app running. It has made electronics accessible to a lot of people. Raspi opens a different world as you have a bunch of other peripherals available for you already builtin, without having to get "shields". There is a large ecosystem of shields available in the arduino world that make prototyping easy. So it may eventually boil down to what you want to do and how much effort you are willing to put in for the project. In my opinion, arduino are good for initial protoyping/weekend projects, and microcontrollers/embedded linux are good for engineering projects and will set you up for a long haul in systems engineering.