One reason would be the location of the motor, and whether the combo will fit with the motor adjustment slots available. Your two examples end up with significantly different motor positions.
Generally in my off road rides I try to keep to the small side on both gears that give the ratio I want and will fit. The motor is then farther forward, farther away from rear guard, thus better protected. The weight/mass of the motor is closer to the center of gravity, which will make the car turn slightly easier. And the smaller pinion will accelerate slightly easier. Not claiming huge differences here, but why not take what ya can?