When you're baking, if has to be butter! I remember the one and only time I made chocolate chip cookies with margarine instead of butter, thinking the substitution wouldn't matter. Boy, was I WRONG! I had paper thin, plate size cookies instead of nice, fluffy cookies. Too much moisture in the margarine to accurately sub it for butter.
Butter only, especially after some of the releases of the alters for ingredients in the margarine. I always knew it was supposed to be 1 ingredient away from plastic lol but now there are health issues. Yuck. I like the taste of butter more anyway