It depends on your location. In some places, PRINCE2 is unheard of, and thus, if you have a PRINCE2 there, then don't expect a better salary than that of a PMP.
I personally noticed that in the majority of cases, the PMP offers a bigger bang for the buck and you are more likely to get a bigger salary with a PMP than a PRINCE2. There might be the odd case though, where PRINCE2 is better.
Just study the market where you want to work and see which one is more preferred: PRINCE2 or the PMP, and make your decision based on your findings.