With a torque sensor, the power delivered is increased proportionately to the amount of pedal force the rider is applying. So as you pedal harder, the motor automatically delivers more assistance. As you reduce pressure, you get a little less assistance. It’s essentially amplifying whatever power you are applying to the pedals. You have multiple levels of pedal assist, each representing a higher or lower amplification of your power. A torque sensor can feel more like riding a conventional bicycle than a cadence sensor. It also tends to deliver power smoother.
A cadence sensor, perhaps more appropriately called a crank sensor, delivers a uniform amount of assistance at each assist level, regardless of the pressure you apply. It is activated just by getting the crank turning. Because a cadence sensor is not reading your pedal pressure, the power delivery is not quite as smooth or “bike-like.” But it’s fairly easy to adapt your use of the controls to smooth out the power delivery. Some people prefer a cadence sensor because it provides a great power sensation without applying much pedal pressure.
The best way to know which pedal assist type is right for you is to try them both.