I got the Garmin Edge 500 installed for my road bike with the bontrager speed/cadence sensor. For my mountain bike, I just need speed. That is the second bike.

Online I see that some people use the GPS feature (no magnet) and others use a speed/cadence sensor for mountain bikes. I am thinking the speed sensor might be more reliable in the woods when I am mountain biking.

Is there really a big difference in the speed measurement with GPS only versus speed sensor? If speed sensor is the way to go- what ant+ unit to use? I see Garmin does not have a single speed sensor unit.

Thanks.

David