It depends on what you want. Is this something you want just for biking or are you going to be running/swimming/etc as well? I have a garmin 310xt which is very similar to the Edge 500 but the edge is just for bike. The Garmin Edge 705 is the flagship model that lets you get the turn by turn stuff.

If you are serious about training, all you need is a basic computer and a heart rate monitor. If you want all out data I would go with the Garmin 310xt or Edge 705. Polar was not recommended to me by the LBS and one of my friends has some issues. Plus the Garmin gives you ANT+ capabilities. Again...Garmin user and very happy!