At the highest level, Garmin and Polar are neck and neck. Garmin's Forerunner 910XT makes the most sense for a serious triathlete. It has a swimming focus, with water-resistance to 50 meters and a suite of swimming-specific features, like the ability to track pool lengths and even understand what stroke you’re using. But it’s also a very capable dry-land unit, with GPS, and the ability to measure pace, heart rate, distance, elevation, on or off of a bike.
Polar's GPS-enabled RCX5 doesn’t have quite the same swimming-specific features as the 910XT, but it still holds its own in a pool. It's also the best of the bunch for measuring your performances, competing against previous best efforts, and recording and tracking your fitness.
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