Margarine is made from vegetable oils. These oils are created in a caustic chemical concoction and the resulting fats are found no where in nature. These fats are structured in a way that the human body is unable to handle well and lead to inflammation.
Butter, especially that made from the milk of grass fed cows, contains much more healthy fats. You can argue over how much fat one should take in, but between the two, butter is the only choice that makes sense for your cardiovascular health.