Learners are expected to discuss and debate, using scientific evidence, the Socio-Scientific Issues (SSI) that often overlap with personal experiences and ethical dilemmas. This study investigated the reasoning of 24 Grade 8 Lebanese students when arguing about a scientific scenario as opposed to an SSI. It also examined how students make decisions and reason about SSIs. Results showed that students' reasoning is more emotional in socio-scientific contexts than in scientific ones. Also, scientific reasoning was based on macroscopic (observed) characteristics of the phenomenon in an SSI but was based on microscopic (cellular/molecular details of mechanisms) features of the phenomenon in scientific contexts. Cultural, social, and ethical considerations influenced students' decisions and reasons when constructing arguments. We discuss the results in light of the literature on reasoning about SSIs and provide instructional implications on how to teach argumentation within SSIs to enhance student reasoning.