I have an Arduino Uno, Raspberry Pi, and a UDOO Quad(4 Core ARM + Arduino DUE on one board)
I made lots of LED controllers with my Arduino and I also made a small machine that could map out a room using an ultrasonic sensor.
You can spend A LOT on the arduino, accessories are pretty expensive, especially the add on boards.
Didn't do too much with the Pi, it was a bit too slow for a mini media server.
I just got the Udoo a couple weeks ago, but I have been wayyy too busy to mess around with it.