Which is a better measure of polarising power of a cation?

Recently, I have come across literature stating that polarising power of a cation is proportional to the inverse of the surface area $(4\pi r^{2})$ while some associate it with the charge density of the cation, and thus state that it is proportional to the inverse of the volume $(\frac 43 \pi r^{3})$. I have been wondering which is a better measure of the polarising power. My knowledge of physics is limited so I would like to ask if anyone could kindly enlighten me on the matter. 


