These two exercises are very different from each other and have a different set of health benefits. Both of them work on your abdominal muscles and reduce fat, but plank works on other muscles as well, while crunches target only your abs. According to a 2013 study, plank with hand reach can target 20 per cent more rectus abdominis compared to a crunch. A plank with a knee lift to the opposite side of the chest activates rectus abdominis activation by 30 per cent and oblique by 20 per cent as compared to crunches.
The clear winner, in this case, is the plank.
However, a plank is a more complex exercise than crunches, so the risk of injury, in this case, can be high.