Electron correlation effects for the Si(111)-3 x 3 and Sn/Ge(111)-3 x 3 reconstructions are analysed introducing 2-dimensional (2D) Hamiltonians whose parameters are deduced from DFT calculations. We find that these two surfaces present striking similarities, with two dangling bonds and one electron per unit cell defining the surface states properties around the Fermi level. Upon introduction of electron correlation effects, the Sn/Ge(111)-3 x 3 remains metallic, while the Si(111)-3 x 3 is found to be semiconducting, mainly due to a kind of charge transfer metal-insulator transition associated with the faulted-unfaulted asymmetry of the surface.