In the field of medical first responder training, the choice of training modality is crucial for skill retention and real-world application. This study introduces the Green Manikin, an advanced Mixed Reality (MR) tool, conceptually combining the immersiveness of Virtual Reality (VR) with the tangibility of real-world training, and compares it against traditional real-world simulations and VR training. Our findings indicate that MR and real-world settings excel in Self and Social Presence, and in intention to use, offering heightened psychological presence suitable for complex training scenarios. Effort expectancy was highest in real-world environments, suggesting their ease of use for basic skill acquisition. This nuanced understanding allows for better tailoring of training modalities to specific educational objectives. Our research validates the utility of MR and offers a framework for selecting the most effective training environment for different learning outcomes in medical first responder training.