Generally margarine is considered to be healthier for you than natural butter, simply because it has little to not fat in it. Although it may be true that butter has no carbohydrates or trans-fat as is frequently advertised on packaging, it does contain a lot of saturated fat. Butter is made up primarily of animal fat which is why it is so fattening and ultimately unhealthy, especially when consumed frequently and in large quantities.
Margarine is usually a safer choice for those who are on a diet and want to stay as healthy as possible. Another benefit of buying margarine at the grocery store rather than butter is that the former tends to be much cheaper and there are many more healthy varieties.