Theoretical investigations of the magneto-structural correlations of M2AgF4 (M = Na-Cs) compounds show that they adopt two polymorphs, the layered perovskite and post-perovskite structures, which differ greatly in the connectivity of the Ag/F sub-lattice and hence in their magnetic properties. With the use of the DFT+U method, the relative stabilities of various M2AgF4 phases were established and the collective JT effect within the Ag/F sub-lattice of these systems was modelled. Calculations show that for all studied stoichiometries, the preferred scenario of the collective JT effect in the layered perovskite phase corresponds to an antiferrodistortive order of elongated octahedra, which leads to 2D ferromagnetic coupling, in agreement with the experimental findings for the M = Cs, and Rb systems. The layered perovskite phase is found to be progressively destabilized with respect to the post-perovskite structure when moving from Cs to Na, again in agreement with the experimental findings. Our results strongly indicate that the layered polymorph of K2AgF4 should not exhibit a ferrodistortive order of compressed octahedra, which contradicts the previous experimental results.