We've all hashed through the argument, thousands of times. The fact is, less of both is better for you, which is part of the HealthiFood program. However, you will notice that some of my recipes use margarine or butter and that I don't care which. Here's why.
I was raised on butter. I feel better about because it is much more natural. However, margarine is a lot cheaper and has (a few) less calories so I use it sometimes. I don't use shortening much (as you can probably tell), and in most things I like to replace margarine and butter (after reducing the amount) with canola or olive oil.
So margarine or butter? In all honesty, if my recipe calls for it I most often use margarine to save money, unless I really want a wonderful taste (and am willing to pay for it) with butter. There are just a few recipes that I always make with 100% butter only: my mom's recipes for cornbread, sugar cookies, and snicker-doodles.