Another great reason for the Raspberry Pi is Google's Coder project.
http://googlecreativelab.github.io/coder/I like to use the Arduino to teach kids and not-kids to program and about electronics. Most recently using the johnny-five library
https://github.com/rwaldron/johnny-five. Physical reward for code effort is really valuable. We've flown quadcopters with and driven a Sphero around with a Leap Motion. You can build a robot with a couple of servos and an Arduino, then program it to drive around, much like the Turtle graphics program I remember getting me into computing way back. Except this time you maybe also built the robot you're driving around with the code you just wrote.
RPi to teach electronics is a bit harder but can be done. The GPIO is not as simple as the Arduino is, though you can make it easier to use with some of the shields that are out there. I've used it for projects that needed only a couple of pins, and it was great, much nicer than the Arduino's event loop.
I'm a huge fan of javascript as a choice language to get kids learning - loose typing, forgiving, and very capable thanks to the efforts of the open source community. You can start with something like Coder, learn javascript for the front end, then move to Node.js for server code. With platforms like Appcelerator you can then use your javascript skills to make a mobile app for Android and iOS as well as the web. I'll be working with some young home school kids next year to see what works best. We'll be starting with Scratch, then maybe Coder. What path we take past Scratch will depend on the kids though. Biggest constraint is going to be tool availability. Not everyone can afford a Pi. But you can go to the library to use Scratch online. And not everyone who can afford a Pi has a spare kb and monitor kicking around the house. But with Coder you program it from the computer you already have, and start learning about web servers while you do it.
TL;DR : Get an Arduino or a Pi and learn Javascript to build cool things that work in the physical world using a language that is widely used.