In short, the exerspy for Body Media armband is more sophisticated than other devices because it uses more sensors to collect data about you. This also makes it more accurate. Below is a graphic that shows how the different sensors work.
The exerspy for Body Media armband is ideal for weight control because it's able to accurately interpret lifestyle activities, not just exercise, and it's been clinically validated for accuracy in numerous studies. The armband also tracks steps, minutes of physical activity, and sleep efficiency.
The exerspy for Body Media armband does not track heart rate; while heart rate data is useful for athletic performance, it is not a good indicator of energy expenditure (calories burned).
Download this device comparison chart for more detail.