RFID readers for passive tags suffer from reader-to-reader interference. Mathematical models of reader-to-reader interference can be categorized into single interference and additive interference models. Although it considers only the direct collisions between two readers, the single interference model is commonly adopted since it allows faster simulations. However, the additive interference model is more realistic since it captures the total interference from several readers. In this paper, an analysis of the two models is presented and a comparison between them is conducted according to several evaluation scenarios. Besides, the impact of the different parameters, including path loss exponent, SIR/SINR threshold and noise power, is evaluated for the two models.