Arduino or Pi? You Choose
The Raspberry Pi is much more powerful than an Arduino but it has a few limitations of its own. Its I/O is much more limited, it can’t use common Arduino shields, and it needs a regulated 5V power supply via its USB connection. All those limitations are solved with the PiLeven.
The Freetronics PiLeven is an Arduino-compatible board based on the Arduino Uno, but with a few changes. Obviously it’s a bit of a strange shape! The PiLeven fits right on top of a Raspberry Pi (either model B or B+) using the Raspberry Pi expansion headers.
For more detail: Arduino or Pi? You Choose