With rack and pinion steering mechanism You loose maneuverability, compared to differential steering. As differential steering bot (such as Stampy) can spin on the spot, it makes it easy to track/follow object, because there is no need to make partial/full 3 point turns in order to approach detected target. This is not possible with rack and pinion steering mechanism, so following object becomes more complex task which has to be dealt with in software. Also, software orchestrating Stampy drives 2 motors, hence steering is achieved by changing PWM ratio between them, where with steering system You want, there is only 1 servo to do all the work, so turn_left(), turn_right() code from Stampy will not drive robot the same way in this case, hence some code will have to be re-written.
Can I ask, what exactly are You planning to use Your robot for?