The RaspberryPi is fun, but I really want to dig deeper into what is happening with the processor. The Arduino is another step closer. It is in the microcontroller category, so there is much more knowledge about the inner workings required than for the Pi. Arduino also has a great community, and an IDE that is easy to install. I haven't done as much with these yet, but I have both an UNO and a MEGA board now for projects in the future.