Stryd captures your real-time running motion with thousands of samples each second. It offers the best responsiveness, accuracy, and consistency. It provides not only your running intensity quantified by power but also your run form related values to help you understand the reasons behind the power change, running effectiveness, muscular fatigue, etc. It works in both indoor and outdoor environments.
Watch power is as good as the quality of the GPS signal at that moment. It only works for outdoor, the accuracy suffers when there is bad weather, running under trees, running through high rise buildings, running on the track, etc. It has a much bigger delay, which may lead to inaccurate or wrong interpretation of workout with highly dynamic speed (e.g. interval)"