Both are different and you need to understand pros and cons… and what you are measuring.
Speed sensor will multiply the number of revolutions with the wheel circumference.
For the sake of simplicity if a wheel circumference is 2 meters it will turn 10000 in a 20km ride. If your wheel circumference is wrong by 1mm you have an error of 10 meters per 20km. You can realistically achieve better than 50m per 20km which in my opinion is pretty good. The trick is to have a good circumference measurement.
Keep in mind that the speed sensor measure a 3D distance - the distance travelled by the wheel.
The GNSS (aka GPS) is a different type of measurement. The first consideration is to know what distance is measuring (you will find this difficult to nail it in most cases) is it a 2D, 3D? How often is the sampling? What’s the receiver precision?
GPS is also prone to bad reception and notoriously bad estimating your vertical position.
When correctly used the speed sensor is simpler and more precise IMHO. GPS is equivalent in the open sky (no valleys, trees, tunnels…) and in a fairly straight route (the boring ones).