A dynamical freezing, which is the analogous of a glass transition, is observed at large concentrations in aqueous dispersions of maghemite nanoparticles. We study experimentally the structure and the dynamical properties of two dense ferroglass-formers in two very distinct states of interparticle interaction, either strongly repulsive or attractive. The static structure of the magnetic colloidal dispersions is probed by means of Small Angle X-ray Scattering both in and without the presence of an external magnetic field. Translational dynamics of the repulsive glassformer are investigated by X ray Photon Correlation Spectroscopy. Slow dynamics and aging properties, which both become anisotropic under an applied field, are here investigated.