Both would work, it depends on your width’s and if your body has a background image – there are so many factors. I normally let the design dictate the layout… if you are going for a magazine style layout with more text than images then a grid style layout would be recomended.
They are really identical, it totally depends on design. If you need a background for that middle area, you’ll need B. You’ll also need B if you are into the "easy-clearing" method of clearing floats (assuming the main content is floated). With A you would need to use a clearning DIV after the main content and sidebar.