it’s not a marginal difference - you gain 50% more available speed by going from 12s to 18s. You can also increase the gear ratio on the 18s board to increase the amount of force at the wheels by up to 50%.
Put simply your 18s board can have the same top speed and 50% more force, or you can have the same amount of force in a board that goes 50% faster than the 12s board.
If you want to understand how much more power/speed you gain by going to 18s, play with the ultimate esk8 calculator.