All in all speed sensors are more accurate. The benefit of GPS speed is: You do not need to measure the wheelsize so this omits one potential mistake.
However, with our speed sensors you are independent from clouds or bridges so no hassle about the GPS signal. Speed sensors also react a lot (!) faster on acceleration or braking in any conditions.
Most of all you will recognize the benefit of speed sensors on slow and steep uphill trails: your speed sensor relies only on the frequenzy of signals from the magnet so the distance covered does not interfere with altitude.