While many believe margarine to be a healthy alternative to butter (mainly because of the high cholesterol in butter I suppose), margarine has several elements that completely negate any health benefit obtained from cutting out the cholesterol.
First off, there are two types of cholesterol: LDL (bad) and HDL (good). When one consumes butter both varieties of cholesterol are ingested. Margarine, while it does contain less LDL cholesterol, also contains less HDL cholesterol which our bodies need.
Second, margarine contains a type of fat known as trans fatty acids. This is a kind of fat that the human body has a great deal of trouble breaking down due to its chemical structure. They are also known to be an important contributing factor to heart disease and high blood cholesterol levels. This type of fat is not present in butter. Butter contains only natural fats that are more easily broken down by our bodies. So while the total fat level of butter may be higher, it is in fact healthier due to the absence of trans-fats.
It is important to consider that when I talk about margarine I am referring to the most popular variety of margarine on the market: those with hydrogenated oils (where the trans-fats come from). It is possible to get margarine without this type of oil (Becel is one). While it may not be as solid at room temperature, it's a hell of a lot better for you.
Anyway, I suggest that you check out the label on whichever brand of margarine you plan on buying next time at the store. Try to get one with no hydrogenated oils and which is low in saturated fat.