Polar FT4 vs BodyMedia

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Today for the first time I wore both to the Gym trying to get the most accurate number of calories burned during the Zumba class and
the Elliptical trainer, to my surprise, according to the Polar monitor I burned about half the amount of calories the BodyMedia calculated.
Half an hour on the elliptical machine: Polar: 108 - BodyMedia: 175
An hour Zumba class: Polar: 250 - BodyMedia: 425
Which one should I go by? Does anyone has an experience with this?

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