If you can pick up a 305 I HIGHLY recommend it! I have had mine for 2 trouble-free years. I adore it for running and with the wireless cadence/speed sensor it has made a bike computer obsolete. They are dirt cheap now, too. Someone was offering it recently for something like $115. I think I paid about 2x that 2 years ago.

As nifty as the 310xt is, I can't justify the price, since I don't swim or do workouts/races of more than 5 hours. The battery life has never been an issue. If it died today I'd pick up another one, no question.