The effective capacitance is 3 time more in delta as compared with star connection provided that same capacitors are used.
Delta impedance is three times larger than star impedance provided all impedance have same value Z_delta = 3 * (Z_star)
It is clear that either you connect capacitor banks in star or delta, they both provide same reactive power to improve power factor. But capacitor connected in delta will have three times larger reactance as compared to capacitance reactance present in star. As capacitor capacitance is inversely related to reactance. So capacitors bank connected in delta will have capacitance three times lesser than star. That economically better.