I used to think that circles are more natural and smoother than boxes, but then I realized, drawing rectangles with the mouse actually allows you to easily control the dimensions for both the y and x axis, so it gives you more degrees of freedom than just circles where you can only control the radius.
You can argue that the dimensionality of a circle area command actually have 3 dimensions of input. Center of circle in x and y dimension and radius as it's own input dimension.
However, a rectangle still beats it with 4 dimensions of input considering it has start x, start y, end x and end y in dimensions of input.