What robot shield's for arduino do you use and why did you choose it?
For robotics, to facilitate interfacing to servos and sensors like Ping sonars, look for
something with a bunch of standard 3-row male headers [signal-power-gnd]. They
are 10X easier to connect than regular Arduino boards.
Picaxe makes a robot shield that is compatible with Arduino: Instant Robot Shield - Hardware - PICAXE
I used a Dagu Red Back Spider controller (based on the Arduino Mega) in my first robot.
http://robosavvy.com/store/product_info.php/manufacturers_id/35/products_id/1574
Then I started designing my own boards with 3A switch mode regulators. I use 2S/3S Lipo batteries to power my robots. The 3A regulator drops the voltage to 5V. This way I can plug in lots of servos and sensors on my board.
My board based on the ATmega328 http://www.bajdi.com/bajduino/bajduino-3a-3/
And my latest board based on the ATmega1284: http://www.bajdi.com/something-different-a-crash-robot/bajduino-mega-3a-2/
I like to use any motor shield. You do not really need anything to interface with sensors, but motors in most cases require either a controller, or a transistor+cap(if you want to risk frying your arduino)
I would go for the motor shield that seems to suit you the most.