When it comes to spreads for your bread, I am a longtime proponent of olive oil. Its heart-healthy fats are a much better choice than margarine or butter.
Margarine was originally developed as a cheap substitute for butter, and has evolved from some fairly unappealing animal-based ingredients into a vegetable-oil based spread with added chemicals that make it more flavorful and easier to spread. To achieve that solid, spreadable consistency, margarine manufacturers hydrogenate vegetable oil, creating unhealthy compounds that may contribute to heart disease and stroke. In addition, the heat and chemicals used to harden vegetable oils produce trans-fatty acids (TFAs), which can contribute to heart disease, increase cancer risks, promote inflammation and accelerate tissue degeneration.
Butter is traditionally made from animal milk and contains saturated fat, but is definitely the better choice. In fact, some recent studies suggest that natural saturated fats may not significantly contribute to cardiovascular disease, though I believe further study is warranted. In any case, butter is closer to a whole food than margarine. If you must opt for a spread that is not olive oil, I suggest natural, organic butter in limited quantities.