Design decisions about computational hardware for small mechatronics project
This question honestly is too broad. You have these criteria:

Suitability of peripherals
Suitability of processing power
Ease of software development
Ease of hardware development

3 and 4 completely dwarf the other two criteria so much when starting out. When starting, just always assume you need a microcontroller in some form until you know for a fact from experience that you need something more.
Arduinos are pre-packaged microcontrollers with software crutches. An RPI is a pre-packaged application processor with an OS. An RPI is an advanced piece of hardware that you have no chance (or money) of building something similar from scratch on your own.
With an Arduino, you are eased into the software side (software crutches in the form of the Arduino programming language/environment) and the bare minimum on the hardware side so you can at least graduate onto bare microcontrollers and re-use the knowledge. You cannot do the same with an RPI because the hardware is too advanced.