Arduino is great bang-for-the-buck, compared to BASICStamp. You can get going with nothing more than a $40 Duemilanove and you are ready to go.
For another $12, you can have the proto shield . . . after that most of the components required for experimentation are already gathering dust in your parts-bin waiting for you.
One of the great things about the Arduino is the open source nature of the beast has helped to spawn off a variety of variations (Freeduino, LilyPad, Nano, Mini, Pro, etc) all at competitive price points. More importantly being open source has encourage the development of a wide variety of shields (proto, motor control, stepper motor, XBee Wireless, etc.) and you can even turbo charge it by replacing the atmega168 with the newer atmega328 chip for next to nothing.
Try that with BASICStamp that costs about $100 to get the equivalent of the basic Arduino Duemilanove at $40.
Having said all that it seems like BASICStamp has a better toehold on the educational market.