From a pure risk mitigation perspective, I think the PLB wins. Loosing your way isn't the only situation you could need one, but injury, unexpected weather or animal attacks are other situations where it could be needed (for example). It gives you a way to get help no matter the emergency.

That said, when facing the same decision I went for the GPS (and I pretty much never have cell service where I go). I ultimately decided that staying found was more important to me than having a lifeline for those less likely, but potentially life threatening, situations.