My research tells me that margarine typically carries about the same number of calories as butter; contains about thirty percent less saturated fat, and 100% less Cholesterol (a Tbsp of butter contains about 30 grams of cholesterol while margarine contains 0%)
Spreads made with vegetable oils typically match the figures for the butter/margarine comparison but contain about 20% fewer calories and about 20% less saturated fat.
Fat free margarine spreads contain no calories, no saturated fat, and no cholesterol. They also contain little or no flavor unless it's chemically introduced.
Now, let's talk about cooking. I use butter where I'm looking for flavor. If the flavor of butter isn't critical, especially in cheese dishes callling for butter or margarine, I use margarine. I avoid those products that come in a tub because I find they have virtually no flavor at all and if all I want to do is lubricate my cookware I'll use margarine. "Spreadable" isn't an issue in my kitchen; I'm trying to make food taste good and I know how to rely on room temperature to achieve the spreadability I'm looking for.
I find it takes less butter to infuse the foods I'm cooking with flavor than it does margarine, and I often add butter as a portion of the margarine measure to enhance flavor. I would never suggest using butter indiscriminately but it has its place in the food preparation process.
