It really all depends on who and when you are asking this question.
Seriously though, the prevailing train of thought is that butter is healthier because margarine contains trans-fats and other artifical fats. The current trend says that these are worse for you than the fats in butter. While I tend to argee with that assessment, who knows. Heck, at one time eggs were considered to be just about the most evil food you could eat, just a few years back. Then there are the studies that show that a high-fat, low carb diet is best then another story that says no, that the high-carb, low-fat diet is the best. I even read at one time that celery was bad for you. So your best bet is to do all things in moderation and take any study, you read, with a grain of salt (though that is bad for you also).