Butter and margarine have a similar of calories and amount of total fat (about 80%), but consist of different types of fat. Butter is mostly saturated fat, where as margarine is either polyunsaturated or monounsaturated fat.
Margarine
Margarine is generally made from polyunsaturated oils (from plants), which means there is no cholesterol present in margarine. Further, these polyunsaturated fats can help lower blood cholesterol levels. Lower fat versions of margarine can be produced by basically mixing the margarine with water which will lower the fat content. In New Zealand, margarines contain little (less than 1%) or no trans-fat.
Butter
Butter is mostly saturated fat, which is implicated in raising blood cholesterol levels. Professor Rod Jackson (Auckland University Epidemiologist) states that butter is the “purest form of saturated fat you can eat and it has no protein and no calcium”.
Source: Sunday Star Times, April 2008.
So what should I choose?
Best advice is to choose a low fat spread such as low fat hummus (less than 10% fat), or Philadelphia extra lite cream cheese, which as 5% fat and is made from cottage cheese so will provide you with protein and calcium.
If you do use butter or margarine, use them in small amounts and spread it thinly!