Either.
There is no such thing as a Single Engine CPL or Multi Engine CPL, Just a Commercial Pilots Licence.
While you will require a Multi Engine Instrument Rating the CPL is the licence allowing you to be employed as a Pilot.
Multi Engine CPL's are generally marketed by organisations that do not have any suitable Single Engine Aircraft.
Do what is best for you, spend more money and gain some Multi Engine hours (Dual) or save the money and do the CPL on a single.
W1