Radio Access Technology (RAT) selection is an essential component in designing heterogeneous network coexistence to enable the diversity of future applications. In the 5G paradigm, many wireless technologies are available to ensure ubiquitous radio access. However, there are still open issues related to the decision-making process and Quality of Service (QoS) constraints consideration when it comes to selecting the most suitable wireless interface for a specific data transmission e.g. in Device-To-Device (D2D) communications. In this paper, we present MCDM-RAT: RAT Selection method between LTE-D2D and IEEE 802.11 formulated as a Multiple Criteria Decision Making (MCDM) problem with a decision data analysis mechanism that allows accurate network performance estimation in the decision making process. Full-stack network simulation results show the benefits of MCDM-RAT compared to baseline decision mechanisms. Our findings show the benefits of MCDM-RAT, under multiple network load conditions, in enhancing network performance perceived by end-users (in terms of delay, through-put, and data delivery rate), while considering simultaneously multiple QoS constraints.