If your wheel circumference is accurate, the cyclometer will be more exact for speed and distance. The Garmin Edges can take measurements every second. In that time, your 700c wheel rotates about 3 times at 15 mph. That means the resolution of the cyclometer is 3 times than that of the GPS. Additionally, the variation in position of the GPS is 3-15 m (much larger than the error in the wheel rotation rate, which is close to zero).

Keep in mind that the GPS first determines position (earth surface coordinates) at different time points and computes speed (and distance) from that. Thus, the speed and distance are secondary results of a GPS. Given that the GPS is a self-contained unit (it doesn't require extra sensors), many people might accept a small reduction in speed accuracy for the convenience.