Neither!
Both are primarily fat and don't have much nutritional value.
Instead, use a light spread of peanut or soy or almond, etc., butter, or use jams, jellies, etc, with no butter/margarine.
Though if you really have to choose between the two, in general, butter is considered better than regular margarine because of the trans fats in margarine, BUT there are some new margarines out there now that are specifically made to not include trans fats and to help reduce cholesterol, and tub margarine is better than block margarine because the latter often has saturated fats and trans fats.