Ask almost anyone in ebike sales and they will tell you ‘torque sensors are more sophisticated/ more expensive/ more better’. It is partially true (i.e. not an overt lie) but it doesn’t mean that a bike with torque sensor is the best for your needs.
Anyway, what’s the difference? A cadence sensor detects if you are pedalling, tells the controller, which then supplies motor power up to the limit you have set on your display. Most cadence sensor bikes also have throttles. A torque sensor bike detects how hard you are pedalling and the controller/motor will then provide a multiple of that pedalling force, from perhaps 50% (Eco) to 300% (Power).
The outcome of that is with a cadence sensor, you only have to pretend to be pedalling to get full power. You could also simply use your throttle. With a torque sensor, you are always helping. Even at the 300% level you are still providing 1/4 of the power yourself.
Perhaps another way to describe it is that a cadence sensor is a constant speed bike (to the limit you set on your display) while a torque sensor is a constant effort bike (the speed will vary if your effort changes).
Now the benefits of the torque sensor bike are:
- Because you are having to put in at least 1/4 of the effort, it will use less battery power and your range will be longer
- It feels more naturally like a bike – you control the speed through your pedals, you don’t have to fiddle with the controls much
But the cadence sensor has benefits too:
- You don’t have to work as hard, so if you want a commuter that you ride in your work gear then you’ll arrive less sweaty
- It is easier to keep at an even speed regardless of effort, and having a throttle gives you extra control for in-between speeds–a bit of boost or when going slowly through a bunch of pedestrians on a shared path or intersection
So, why not have both? The only bikes I know of that do have both are the SmartMotion Pacer/Catalyst which allow you to select which mode to ride in. Smart.
A torque sensor is a more complicated piece equipment so adds cost to the bike (perhaps $100 at manufacture), so the main reason it is sold as ‘better’ is that it is more expensive. Almost all mid-drive bikes will have torque sensors (aftermarket conversions typically don’t). [Edit Jan 2018: This cost has come down considerably, see interview with Wai Won Ching] [July 2018: I learned that there are different ways to implement a torque sensor. Because the strains being measured are tiny, the positioning and materials used have a big impact as does the programming of the firmware, ie it is not as simple as whacking on a cheap measuring thing]
You’ll notice that I haven’t yet answered the headline question. The answer: It depends…
My advice is always: ride the bike further than around the carpark, inform yourself and make your decision from there. And for eMTB a torque sensor is essential.