The cadence sensor reacts to rotation and switches the support on when you are pedaling, to a levels you have chosen. The torque sensor reacts to the pedaling pressure and adapts the amount of support accordingly: the more pressure, the more support. The torque sensor is sometimes preferable to couples when one partner rides an e-bike and the other one a "normal" bike, because the dosing of the support is finer adapted to the needs. If you are riding alone or along another e-bike rider, this is of less importance. Also, the torque sensor makes the bike more effective and increases the range.