When uncertain, medical trainees often seek to co-regulate their learning with supervisors and peers. Evidence suggests they may enact self-regulated learning (SRL) strategies differently when engaged in self- versus co-regulated learning (Co-RL). We compared the impacts of SRL and Co-RL on trainees' acquisition, retention, and preparation for future learning (PFL) of cardiac auscultation skills during simulation-based training. In our two-arm, prospective, non-inferiority trial, we randomly assigned first- and second-year medical students to the SRL (N = 16) or Co-RL conditions (N = 16). Across two learning sessions separated by two-weeks, participants practiced and were assessed in diagnosing simulated cardiac murmurs. We examined diagnostic accuracy and learning trace data across sessions, and conducted semi-structured interviews to explore participants' understandings of their underlying choices and learning strategies. SRL participants' outcomes were non-inferior to Co-RL participants on the immediate post-test and retention test, but not on the PFL assessment (i.e., inconclusive). Analyzing interview transcripts (N = 31) generated three themes: perceived utility of initial learning supports for future learning; SRL strategies and sequencing of murmurs; and perceived control over learning across sessions. Co-RL participants regularly described relinquishing control of learning to supervisors and regaining it when on their own. For some trainees, Co-RL seemed to interfere with their situated and future SRL. We posit that transient clinical training sessions, typical in simulation-based and workplace-based settings, may not allow the ideal processes of Co-RL to unfold between supervisor and trainee. Future research must examine how supervisors and trainees can share accountability to develop the shared mental models that underlie effective Co-RL.