Conceptual inquiry-based introductory physics labs deploy PER-informed pedagogical strategies in physics labs with the aim of helping to improve students' understanding of physics concepts. Unlike traditional labs (which tend to be highly-structured and focus on verification of scientific relationships via precision measurements) and skills-based labs (which tend to eschew the aim of helping students learn physics concepts in favor of teaching experimental skills), some studies have suggested that conceptual inquiry-based labs may have a positive impact on student conceptual understanding, as measured by concept inventories. This paper reports on a randomized controlled study that compares student conceptual gains in electricity and magnetism between a conceptual inquiry-based lab and a skills-based lab. Following a difference-in-differences analytical strategy and using hierarchical linear modeling, the result from this study is that the conceptual inquiry-based lab provides no additional benefit to students' conceptual learning gains compared with the skills-based lab. Studies such as these may help physics departments make decisions about the goals and scope of transformations to their introductory lab courses.