Firstly, a scapular is not a medal. Scapulars and medals are both worn on your neck, but a scapular - being a derivative of a monastic garment - comes in the form of two pieces of cloth tied together by a cord, one piece placed in front and one behind. A medal, on the other hand, is a single round hunk of metal.
There are many different kinds of scapular. The most popular with the brown Scapular of Our Lady of Mount Carmel, which as the name suggests is an item associated with the spirituality of the Carmelite Order. The central devotion behind the scapular is to that of Mary, and it is promised that “those who die wearing this scapular shall not suffer eternal fire.”
You can read more about it here.
The St Benedict Medal is a medal that is associated with the Order of St Benedict. It is a very detailed and specific design that carries symbols and phraseology associated with St Benedict, the father of Western monasticism. Over the centuries, it has also gathered devotions in its efficacy in many situations, most prominently in casting out demons and obtaining the protection of St Benedict.
Which one you choose to wear depends on which appeals to you better. I personally prefer the St Benedict’s Medal because I am in favour of the ancient Benedictine spirituality of work and prayer (where each is essentially a form of the other).
I do not wear both because I cannot claim to have such a skill as to live out both spiritualities and still properly deepen myself in both at the same time. When I wear a sacramental, I make it a point to devote myself to that spirituality of that sacramental, lest it becomes nothing more than a fancy accessory, so to dedicate myself fully to two spiritualities is not something I have the ability to achieve. Also, it makes my neck very congested.
