about characterizing mim capacitors on silicon substrate

MIM capacitors on silicon or other substrate have two metal plates between insulator(now MEMS switches stuctures and tunable capacitors are similar)

in characterizing this kind of device, may be we can use one-port S-parameters or two-port S-parameters measurements and both can be used to derive the C and Q value.
this relationship holds:
S11oneport = S11 - S12*S21/(1+S22)
Sij-two-port S-parameters

but I think maybe they can reach different results, as in many circuit configurations(e.g. filters) MIM caps usually have one metal plated grounded(usually the bottom plate), thus the substrate effects are shielded. Thus, these device cannot be characterized in two-port S measurements. As in this measurement technique, the bottom metal cannot shield and count in severe substrate coupling.

Can you give me some comments on the above point of view.