I love this question. Its a matter of understanding how both butter and margarine are made. Margarine is manufactured from vegetable oil and goes through hydrogenation, which keeps it solid at room temp and gives it a longer shelf life. The reason companies do that is to take out the saturated fat (the bad kind of fat that increase the bad cholesterol). Unfortunately hydrogenation adds TRANS-fat, which is even worse! Your body treats both of those fats as the same! In addition, they also add artificial chemicals and preservatives for flavoring and color.
Butter is more natural, made from churning cow's milk and cream. It does have saturated fat, but no trans fat. If someone has diabetes or high cholesterol, it is best to not eat either one, or at least eat in moderation. In terms of which one is a relatively healthier choice, I would choose organic butter. Organic butter actually still has vitamins A, D, E,K and is made from the healthy cows raised on green fields. And if you still prefer margarine, buy the soft ones in the tub and read the labels. Look for "no trans fat" and "no hydrogenated oil." Avoid the hard stick margarines- those are really high in trans fat. In the end, the problem is eating too much of either one. Hope that helps. :-)