I use the Garmin Forerunner 305. I guess it would depend upon all of you "NEEDS" and "WANTS". The 305 has enough for me and the 310x is supposed to be better for the swimming portion, but not accurate for HRM or tracking in swimming. I use the footpod and the cadence/speed sensor on the 305 for the treadmill and roller workouts.
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2007 Jamis Ventura Comp
2006 Jamis Explorer 2.0
2000 Specialized Hardrock (bought used)
Swim, Bike, Run and sounds like fun