Testing is mainly known as the process of checking that a system possesses a set of desired properties and behavior. Its importance and impact are becoming crucial and strategic for the deployment and use of software and systems. Several techniques have been defined all along the protocol testing process whose formal techniques that allow validating multiple facets of a protocol implementation such as their reliability, scalability, security, and in particular its conformance to the standardized requirements. Among these testing techniques, the ones denoted as passive are currently often applied. Indeed, there are non intrusive and based on network observations. In this paper, we therefore intend to help and guide the protocol testers regarding their testing choices by considering the functional protocol properties to check, and the analysis of testing verdicts obtained by applying passive testing tools. We propose a compared analysis of the application of two efficient passive testing methodologies. The comparison is performed through the study of the Session Initiation Protocol. The results analysis demonstrates that depending on the properties to test, the way to model them, the way of testing (on-line/off-line), the available testing time ressources, tradeoffs are needed. Thus, this analysis aims at guiding the testers when tackling the passive testing of communication protocols