Capacitors are generally used where the reactive loads are fairly constant. Synchronous condensers are used when quick response to reactive demands and the capability of addressing quality issues - such as voltage regulation and flicker. Below is a list of points should be consider in selecting synchronous condensers vs. capacitors from the book, Electrical Transmission and Distribution Reference . Synchronous condensers allow for a finer adjustment to changes in the load. Synchronous condensers can be overloaded for short periods of time, whereas this is not the case for capacitors.
Capacitors have lower losses than synchronous condensers.
Capacitors are less expense than synchronous condensers. Synchronous condensers add to the short-circuit current of a system and may increase the size of the breakers required. Switching capacitor banks can cause transient and harmonic problems.