The Polar and Wahoo armband HRMs were reviewed by dcrainmaker and said to be pretty accurate, with the Polar being about as accurate as as the very accurate Polar chest strap HRM. The early Polar armband had what I'd call insufficient - poor, actually - battery capacity. Neither can do HR variability, so I'd recommend against them, because you might want to work with your HRV in the future, or now for that matter.