The speed sensor is less prone to error (variability) which means speed and distance will be more accurate. But it needs an accurate real wheel circumference.
With the speed sensor, you can get accurate speed in a single wheel rotation (about 6 feet).
The error (variability) of the wheel sensor is fairly close to zero (a few inches).
The error (variability) in the GPS position is more like 3-10 feet (and you can also lose the signal).
So, with the sensor, you get more accurate speed and better response to rapid changes.
GPS is ok as long as you're not riding slowly e. g. at about 2-5 kph (then, the 520 tends to stop recording since it thinks you're staying still). Won't happen often on a roadbike, I guess, but on a mountainbike it does every now and then especially when riding more technical trails with a lot of switchbacks. That's why I use a speed sensor on my main mountainbike.
