Butter is better. Margerine was created as a cheap substitute, and that's exactly what it is. Some claim its chemistry is almost identical to plastic, which wouldn't surprise me.
I'm among those who grew up on margerine, unfortunately, but perhaps I ate little enough to avoid the serious consequences associated with chemical-based food...so far, so good. Butter, on the other hand, is made from real food. It has natural components, some of which probably haven't even been discovered yet, as new nutrients are being discovered all the time. (Who ever heard of antioxidants or phytonutrients 50 years ago?)
But beyond that, the flavor issue seals the deal. Butter is better. Butter contains the natural components of milk, and you can tell if you really stop and savor the flavor.
I suppose vegans could argue that margerine is good for cows. I say we can be good to cows without swearing off the healthful products they provide...we just have to decide to do it. I've heard the argument that humans are the only animal that drink another animal's milk...as if that's a bad thing! Humans do lots of things other animals don't do; that doesn't make it wrong. Milk is actually a nearly perfect food for human beings, nutritionally, and it would be crazy to give it up as a source of nourishment.
...Whereas if margerine ceased production tomorrow, our race would be better off.