Well, they do two different things. There is the scapular, which is a cloth, a vestment. It is a short abbreviation of the monastic habit of the Carmelites, which, by tradition, Our Lady indicated to the Carmelites how to wear and declared as hers in some way this Carmelite habit, which is then reduced to a small symbolic size as a scapular made from cloth.
The other is the Miraculous Medal, which Our Lady also gave and through which she promised graces in both cases. So, you have to decide: either you wear the scapular, or you can wear both.