Collaborative design practices, in which pre-service teachers construct lesson plans in small groups, is regarded as beneficial to attain professional development. However, it is largely unclear which factors determine the effectiveness of collaborative design within technology-related teacher education. Against this background, we investigated to which extent the group composition regarding motivational (self-efficacy, utility-value) and knowledge-based heterogeneity affected the quality of designed lesson plans and outcome of professional development interventions. Furthermore, we investigated whether the quality of the lesson plans was related to pre-service teachers’ professional development such as acquisition of technological-pedagogical-content knowledge (TPACK). The data was collected within a larger research project which investigated the effectiveness of a TPACK-intervention within 5 subject pedagogies to foster pre-service teachers’ learning. Lesson plans of N = 68 pre-service teachers nested within k = 23 natural occurring groups were analyzed regarding their instructional quality and technology exploitation. Additionally, we measured pre-service teachers’ technology-related professional knowledge, and their technology-related motivation in a pre-post-test-design. The analyses revealed that motivational group heterogeneity positively affected the quality of the collaboration product (i.e., lesson plans). The quality of the lesson plans was not related to the acquisition of technology-related professional knowledge, but negatively related to pre-service teachers’ self-efficacy and utility-value. Similarly, the heterogeneity regarding prior knowledge was negatively correlated to TPACK knowledge gain. These findings highlight that heterogenous group composition during collaborative design practices in pre-service teacher education may be a double-edged sword to attain professional development.