Surface plasmon polariton (SPP) excitation effects on the magneto-optical (MO) activity of Au capped Ag/Co/Ag trilayers are studied as a function of Co thickness. An enhancement of the transverse MO Kerr signal under SPP excitation as compared with that obtained without SPP excitation is measured with a maximum value of 150 times obtained for the trilayer with 8 nm Co. Such enhancement on the magneto-optical activity due to SPP excitation is also five times higher than that obtained in Au/Co/Au trilayers in similar conditions. The lower optical absorption in the studied range and the sharper plasmon resonance of Ag vs Au are responsible for these values. On the other hand, magnetic field-induced SPP wavevector modulation ${\left(\Delta k/k\right)}_{\text{SPP}}$ is studied for these trilayers and compared both with previous results in the Au/Co/Au system as well as with the theory. In the wavelength considered here, the obtained values are similar for both Ag- and Au-based structures and on the order of ${10}^{-4}$, pinpointing the role of the magnetic layer on the SPP wave-vector modulation.