I am using a Garmin Forerunner 405 watch style GPS. It has taken me a couple of weeks to get used to the differences in operation between the 305 & the 405, but the more I use it the more I like it.
Obviously, it is a bit more inconvenient to mount the watch style GPS to monitor current speed, pace, heart rate etc, but it is more convenient to have the watch style GPS to use in other forms of cross training as well.
You can preset sessions, have instant feedback on pace, speed, distance & heart rate and it is all easilly reviewed through the Garmin Training Centre application that comes with the device. You need to be prepared if the sessions are more advanced than a basic interval or distance/time plan and set the sessions up in the Garmin Training Centre application.