I am going to purchase the Gamrin Cadence sensor 2 as my trainers estimation for cadence is terrible. The speed on zwift seems accurate however I can get a bundle with a speed and canence sensor. I wonder, would there be any increase of accuracy if I get the bundle with the speed sensor instead of using the speed recorded by the smart trainer?
The only time Zwift uses a speed sensor is if you use it with a classic dumb trainer. This is the only way it can estimate power with those types of trainers.
With a controllable trainer and a power meter of some type, Zwift calculates the speed and will not use a speed sensor.
Power is speed on Zwift (along with their algorithm that has many variables).