I think most triathletes use heart rate monitors, both for training and for racing.
I use the polar 625x. It is a bike and a running speed sensor. So you can use it for both. Disadvantage of this model is that in a triathlon, you can't switch from the bike sensor to the running sensor. First you have to stop time, than switch to the running and start time again in a new file. So you get 2 different files.
In a triathlon I have it on the bike sensor and I don't use the running sensor.
I heard that you can switch between these functions when you have a more expensive Suunto system.
Thanks guys... That brings up a good point... Do you really need the bike features? Wouldn't you train on the bike similarly with a watch that just reads heart rate? rather than focusing on speed, etc...?
For training it is nice to have all things monitored in one place. I too have the Polar s625x. For racing I also have it set to the bike speed and don't worry about the run speed setting. I don't look at in during the race I do hit the lap button when entering trans.
I use heart rate for the bike training. But afterwards I also want to see the specific bike features. So I don't focus on speed and distance, but I do want to know.
I don't need the bike features, it's just some more information for me.
I use a Garmin 305. Love it. Has one feature I find to be excellent upgrade from 301. Multisport function- you can set it up to track your bike, T2 time, and run. Or if doing a DU you can record the whole race. The one down side is that you can't use it in water. Would definitely check it out.