Flexural Vibration of Rings: Plane stress or strain

I have a ring geometry. For example the dimensions are:

radius: 2mm

radial thickness: 30 micron

axial thickness: 100 micron

I am interested in the in-plane flexural vibration of the ring.  I am bit confused which assumption to choose for 2D in-plane analysis. Will it be plane stress or plane strain? When I do an eigenfrequncy analysis (using FE software) I get a significant difference in results between plane stress and strain assumption. With my limited knowldege, I think plane stress assumption should be used. Please can someone help me with this confusion. 

Thank you.