We have numerically investigated the plasmonic refractive index sensor capabilities of Au-Ag- Cu alloys using experimentally determined permittivity values. Calculations were performed by simulating the typical Kretschmann configuration of surface plasmon polariton (SPR) excitation, as well as the localized surface plasmon resonance (LSPR) of nanoparticles. Sensor performance was evaluated as the response to changes in the refractive index of the surrounding environment as well as the attainable separation of peaks. By considering not only the peak shift, but also the spectral separation. SPR and LSPR sensors can be discussed on equal terms. In studies of both SPR and LSPR. specific alloy configurations were found to be superior to pure gold.