Gas Discharge V-Q Lissajous
Just looking at the second curve, the smoother transition in the upper right and lower left is saying that the transition before breakdown and subsequent conduction happens over a broader range, meaning that there  as an earlier onset involving few elements and a few elements that take more to break down.

Looking at your two topologies the glass will have much more abrupt transition because it has much lower dimensionality to the surface.

Ask yourself, what would you see if you put a more textured surface in the glass only cell?  Especially something that has sharp peaks (i.e. something that concentrates the E-Fields)