Basically they are two different types of magnesium. Epsom salts are magnesium sulphate and Ancient Minerals is magnesium chloride.
On the other hand Ancient Minerals Magnesium Chloride is not toxic and is very beneficial to the body.
Magnesium is absorbed from your food but our current diets and food sources are depleted in magnesium. Low Magnesium is associated with heart disease, diabetes and osteoporosis as well as many other common illnesses.
Taking magnesium through the skin is a safe and easy way of increasing your magnesium.
Best wishes in health