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.

Good luck.