We present a comparative study of the structural properties of three different models for the Si(001)-SiO2 interface. The models are obtained by attaching different crystalline forms of SiO2, such as tridymite and beta-cristobalite, to Si(001). In the case of tridymite, the Si bond-density reduction is accounted for by introducing either dimers or oxygen bridges at the interface, whereas the construction proposed by Ohdomari et al, has been used in the case of beta-cristobalite, The models have been allowed to fully relax within density functional theory. None of the models shows electronic states in the Si gap, Compared to the trydimite models, the longer Si-O bonds found at the interface of the beta-cristobalite derived model suggests that the latter is a higher energy structure.