If price isn't an issue, go with the Garmin 310xt. It does everything that the 305 does, plus much more. It's got a GPS and a heart rate monitor, and it's got some neat features unavailable in the other Garmin watches. Also, it's a watch tailored towards the three triathlon sports. I recognize that you don't swim often, but when you do, it can be used to track your swim distance and has a feature to time all three sports smoothly. And unlike the other Forerunners, the 310xt is waterproof to 50 meters as opposed being just water-resistant. I also highly suggest a cadence sensor if you're a frequent cyclist.
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