Margarine is marginally better than butter - but it isn't great
Nutrition researcher Martijn Katan calls butter "the cow's revenge." Made from animal fat, its high saturated fat content raises blood cholesterol levels and increases the risk of heart disease. It should only be used occasionally.
Margarine is better, but only relatively. Although it comes from vegetable oils rather than animal fats, the hydrogenation process used to turn it into a solid creates trans-fatty acids, a hybrid that has been found in recent tests to elevate low-density lipoprotein cholesterol (the so-called "bad" cholesterol) to the same extent as saturated fats. 
What's more, trans-fatty acids appear to lower high-density lipoprotein cholesterol ("good" cholesterol), thereby limiting one of the body's best natural defenses against heart disease.
Even so, margarine remains healthier than butter because it contains no dietary cholesterol and has a lower overall percentage of cholesterol-raising fats. Tub or squeeze varieties are the healthiest because they are the least hydrogenated and so have the least trans-fatty acids.
Healthiest may not seem so desirable if you're using soft margarine to bake cookies. To keep fat content down, these spreads substitute water for fat. But cookie makers have discovered that the more watery spreads make cookies spread while baking and give them a flabby texture and bland taste. To avoid such untoward results, the fat content should be 60 percent or higher, say baking experts. Check the package for percentage fat.
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