I would think that of the two, the Polar would be a more accurate means of measuring one's caloric expenditure as it has the added HRM feature. But remember all these devices--Nike+ sportband, Garmin, Polar, Fitbit, even apps, use an algorithm using your personal data to base your calories expended.
That being said, as a runner, when it comes to speed work (tempos, intervals) and long, slow distance runs, my Garmin is the be-all, end-all when it come to my running improvement as I am able to track my pace. But you can use the Nike+ app and get similar data.
Just my take...
Edited by: SP_COACH_NANCY at: 1/11/2013 (21:23)