The Introductory Mechanics course at Colorado School of Mines uses a hybrid lecture-studio model. In studio, students work in groups of three through scaffolded problems and experiments for two hours twice per week. Research shows that rich learning can take place in small peer group settings with appropriately designed activities, but it is unclear to what extent group composition in terms of ability or gender impacts physics learning. To explore this question, we assigned half of each studio class to groups with mixed incoming physics ability (using FMCE pre-scores), and half to groups with matched incoming physics ability and gender. We evaluate the performance of each group type according to the students' scores on the FMCE (pre and post), common course exams, the CLASS, and a survey about their studio groups. We found neither composition type superior by any measure; but did learn that pace is a concern, group communication is sometimes limited, and that only 1% of students request a different group than assigned.