I wanted a wrist only HRM but asked around and everyone I spoke to advised me against it.
Apparently they are more uncomfortable- you need to wear then tightly to ensure a reading and even then the reading is not as accurate as a chest straped HRM.
Added to that with a chest stap version, you can wash it every time you use it in a washing machine (just make sure you follow the washing instructions and remove the battery...) but with a wrist version you cannot do this which makes me wonder just how uncomfortable the strap must be.
With the current HRM I have I have the strap on lose enough that my hand is not blue, but this means it can still move around a little. This would cause a problem to a wrist only version as it would be easy for it to lose its contact with the vein and so lose the heart rate which would be annoying if you were trying to keep tabs on your workout.
Maybe ask around, try various websites and forums, theres a great HRM website attached to the runners world website, even if your not in the UK and just want advice they are really helpful.
