Have you looked at the Garmin Edge 500? It won't map like the 705, but it will do a breadcrumb trail and give you compass and directional info. It also has a more accurate HRM based calorie counter and costs a good chunk less. I don't know if the Polar unit you were looking at was gps based or not, but I bought the Edge 500 specifically because I have several bikes and I can mount it on any of them and go and have all features except cadence which I only have the sensor on one bike. Here is a (possibly overly) in-depth review of it:

http://www.dcrainmaker.com/2009/11/g...th-review.html