Suitable processor for a Robocup Junior Soccer robot?
Consider using the Parallax Propeller and dedicated motor controllers such as the DMC-1. The Propeller has 8 32 bit cores which can run up to 8 processes simultaneously. You can program it using its native OO language SPIN, or for more performance, its own assembly language. 

The DMC-1 is a low cost I2C dual motor controller which can drive loads of up to 2.6A at 15V continuously. It's very easy to control. You can find out about it here: http://web4robot.com

Even if you decide to use the Arduino, I would recommend using the DMC-1.