Specification wise Raspberry Pi is better but for building a robot it’s totally dependent on the requirement of that specific robot. For example, if you using few motors and actuators with 3–4 sensors then arduino is more than enough to handle all these things but when you gonna use camera, opencv or other high computational sensors then RasPi is better. read more
If your robotic application depends on high end image processing and computation then Raspberry pi is the better option. It also depends on the type of motor you will use. If servo motor is used in the robotic application then Arduino can be used. Raspberry pi sucks at controlling multiple motors, especially if the control signals are PWM. read more
An Arduino is a microcontroller motherboard. A microcontroller is a simple computer that can run one program at a time, over and over again. It is very easy to use. A Raspberry Pi is a general-purpose computer, usually with a Linux operating system, and the ability to run multiple programs. It is more complicated to use than an Arduino. read more