The differential configuration (often referred to as a balanced line) is used when better noise immunity and improved timing are required. In differential mode the signal and its logical complement are applied to the load.
The balanced line thus has two signal conductors and an associated reference plane or planes as in the equivalent single-ended (unbalanced) case. Fields generated in the balanced line will tend to cancel each other, so EMI and RFI will be lower than with the unbalanced line. External noise will be "common-moded out" as it will be equally sensed by both signal lines.
Note that in the following diagrams (except the Broadside-coupled Stripline) the traces are trapezoidal in profile and width, W, refers to the trace width nearest the upper surface, W1 refers to the trace width nearest the lower surface.
Edge-coupled Surface Microstrip
Edge-coupled Coated Microstrip