Microstrip is a nearly TEM transmission line with two conductors that can transmit from DC to very high frequency, but with increasing loss at higher frequencies.

SSPP is a slow wave structure with metal patterning on the surface of a dielectric where the field travels at the interface between two materials. It cannot transmit DC. The wavelength is much less than free space so the fields cannot radiate unless they are coupled to special structures to generate variations with wavelength similar to free space.

https://ceie.szu.edu.cn/heyejun/paper/C89.pdf

I agree with Malcolm White's.