If a horizontal disc hits an object equal forces are appled to the disc and the object (Newtons laws etc,) hence Hypno Disc throwing its self sideways and breaking down on a number of occasions.
When a vertical (spinning upwards) hits an object much more of the force is applied to the object as the robot cannot move downwards (assuming its on a strong floor).
So in terms of effect vertical wins but as has been mentioned steering it can be a problem.