When it comes to performance, there's a world of difference between the Raspberry Pi and Arduino. The former, after all, is technically a fully functional computer that can run games, word processors, ...
I'm a fan of Lego robotics. I've written over half a dozen books on the Lego Mindstorms NXT robotic system, and tinkering and prototyping with the NXT just never gets old. (And there's a NEW system ...