Heart rate monitors like Polar and CardioSport have been the most used strategy in triathlons because these events have such long durations. If you like to use a heart rate monitor for pacing, triathlon and especially ultra distance triathlons are definitively the place to wear heart rate monitors.
It is clear that when a constant tempo is the ideal pace in a race situation, then there is a good chance that your heart rate will show you how tough you current workload is. In cycling races the workload (Watts) is very inconsistent (sprints, jumps, hills, breaking, corners, etc.). In a traditional triathlon race there is lot more consistency in the power output, thus there is a lot better correlation between heart rate and workload.
Power Meters to the People
Power meters like SRM, Ergomo and PowerTap are very common among pro triathletes, but actually they would be at least as helpful for beginner triathletes.
When a triathlete enters a competition for the first time, he will realize that it is very difficult to get an efficient pacing strategy. It is very tempting to hunt down the opponent in front of him and it is difficult to save the energy for the final part of the race (hint: there is a run AFTER the bike race in triathlons…)
Even though there is a better correlation between heart rate and workload in triathlons, there is really good arguments to start considering power meter training. By knowing your critical power outputs for the distances you compete in, you have the best opportunity to optimize your training program.
As an example I would like to mention Dave Simonson who participate in the power meter project. He has gained several watts because he has learned how to pace himself and trained at the right intensity.
Heart rate monitors are good for pacing, but they are still influenced by several external factors (temperature, caffeine, level of recovery, etc.) This indicates that there is a great potential for beginner triathletes to improve their performance.
Power meter training improves their skill for pacing and that is one of the most important non-physiological skills in triathlon