In this paper we address the problem of comparing access control models. Indeed, many access control models can be found in the literature and in order to choose one model for a particular context, some tools helping such a choice are needed. We develop here a complete example allowing to compare (in a formal way) the Bell and LaPadula (BLP) model and the role-based (RBAC) model. In order to achieve this goal, we first express these models in a uniform way, then we introduce concepts (mostly based on simulations) allowing to compare access control models.