The Plank Exercise is frequently used by many personal trainers to build core strength. But it also is effective for weightbearing through the wrist and forearm (important to people with osteoporosis to reduce the risk of fracture in case of a fall) as well as developing shoulder stabilization.
Remember that your exercise program should reflect your fitness goals and capabilities. In the case of the plank exercise, if your goal is core abdominal strength then you should choose the lower angle position. If your goal is to develop shoulder strength and reduce your risk of fracture from a forward fall, then the elevated plank stance is probably more appropriate.