Margarine is loaded (LOADED) with trans fats. Butter has none. Butter tastes better. Butter clearly FTW
i think butter makes you have higher cholesterol though, even if it does "taste" better. 
Trans fats are, like, the worst thing evAr for your cholesterol. Not only do they raise your bad chol. they lower the good. They are prolly the worst thing you can put in your body (high frutcose corn syrup being the second worst). Anything that says "partially hydrogenated" on the label you should not buy.
But, I actually just looked it up and recent news stuff says the margarine makers are starting to eliminate the trans fats so margarine may actually start being not as bad for you as butter.