Power data is most useful when you are training for specific zones or as check when you are going to hard/easy. Of course, this requires a ant+ power meter but

Power data is most useful when you are training for specific zones or as check when you are going to hard/easy. Of course, this requires a ant+ power meter but these are getting cheaper these days. Probably the best upgrade is a lower end pedal based power meter. Heart rate, again, I find more useful for training, but in some cases can indicate fatigue that you may not actually feel in the moment.