Im assuming you mean bi-polar stepping motor.
There are advantages and disadvantages to both.
A servo has absolute positioning system, whereas a unmodified stepper motor has only relative position (if you count the number of steps).
A stepper motor has continuous rotation, and unmodified servo has a limited range.
A servo has its drive electronics onboard, you standard stepper motor doesnt.
A servo has a larger torque/size than a stepper.
To control them check google -
here are a couple of links http://www.piclist.com/techref/piclist/jal/DrivingBipolarStepperMotors.htmhttp://www.ams2000.com/pdf/step101.pdf
PS - Unipolar steppers are much easier to drive.