The whipped form has air whipped into it to make it softer and more spreadable. When measured tablespoon for tablespoon, the whipped form is actually less in weight than the stick form so it would not be an exact substitute.
Also, when substituting margarine for butter, use only margarine made with 80% fat. Products with less than 80% fat will have a negative affect on the quality and texture of the end product. See Ingredient Tips
for more information on butter and other ingredients.