I like feedback on this. Been experimenting with my Garmin 500. Comparing speed. I have used the Speed sensor magnet to measure speed and also just took it off and let speed be determined from GPS. I don't find much of a difference. Just a longer lag with calculating speed when from a stopped position.
How much more accurate is using a speed sensor really?
Accuracy of the GPS depends a lot on conditions. If you ride through forests or canyons or cities with tall buildings, GPS can be unreliable. In perfect conditions, the GPS is probably accurate enough for most people.
Assuming you have the correct tire diameter entered into the computer, the speed sensor will be more accurate and reliable, as mentioned above. Also, it you use a training stand, only the speed sensor will work.