Short Answer: Always collect the Dipole-dipole array AND Strong-Gradient array.
Long Answer: As a best practice for 2D surveys, you should always collect Dipole-dipole and the Strong Gradient Array to reduce filtering out important features in the ground. Electrical imaging surveys are made to explore the subsurface. Resolution is enhanced if you collect both arrays because the potential field is measured in between the two transmitting electrodes as well as surrounding the two transmitting electrodes.
The Dipole-dipole array is the primary array for any high-resolution survey. Adding the Strong-Gradient array (the Wenner and Schlumberger arrays can be computed from the Strong Gradient array results; it is also well suited for newer and faster multi-channel systems) assures complete data coverage and higher signal levels which helps when some dipole-dipole may not be retained in the final model due to misfit filtering/noise.
The Strong-Gradient is a much faster array to collect (80% faster) than the Dipole-Dipole array and has higher signal levels, but lower resolution. It is standard practice for 2D surveys to collect the Strong-Gradient in a merged or hybrid command file of Dipole-Dipole + Strong-Gradient. If a merged command file is not available, you can always collect Dipole-dipole first and then the Strong-Gradient second from their respective command files and merge the resulting data files later in EarthImager™ 2D.
For more information on how to merge command files, see this article: How do I make a combined command file?