Insufficient physical activity is a major challenge in our aging society. Although exergames can provide enjoyable exercise opportunities for older adults, it remains unclear which display technology is best suited to reach this goal. This paper compares two popular exergame technologies with different immersion levels: (i) a virtual reality head-mounted display (VR-HMD) and (ii) the ExerCube, a commercial projection-based system. We conducted a within-participants study (N=34) with older adults to evaluate player experience, presence, cybersickness, game performance, and physical exertion. Both display types provided a comparably high player experience and physical exertion that can benefit older adults' physical well-being. The VR-HMD offered superior presence, while the ExerCube led to higher performance and physical activity. Our findings advance the understanding of how different exergame technologies affect older adults' experiences. We present research and design implications to guide the future development of age-appropriate exergames.