I'm about to buy speed and cadence sensor but I'm wondering witch type of sensor is the best for indoor cycling the magnetic or the non magnetic? What's the difference between them?
It looks like they both should work for indoor. They both connect via Bluetooth or ANT+, the main difference is how they're attached to the bike.
The combined sensor is attached to the chainstay with one magnet attached to the left crank arm (for cadence) and the other on a spoke (for speed).
The individual sensors are neater. Nothing goes on your chainstay, instead the cadence sensor goes on your left crank arm (or shoe), and the speed sensor on either front or rear hub (rear if you're using a turbo trainer).
Have a look at some unboxing/setup videos on YouTube. If the bike is going to live on the trainer then you won't have to fiddle around with magnet placement every time you change a wheel, or worry about the unit getting knocked off the chainstay. If you plan to take the bike out though I'd get the individual sensors.
The speed sensor doesn't really add much for indoor cycling, but if you want the two, look for the bundle https://eu.wahoofitness.com/devices/bike-sensors/speed-and-cadence-sensors-bundle as it saves €20. I've both types on different bikes (albeit Garmin equivalents) and the non-magnetic ones are a lot neater. Functionally though it's much of a muchness
The magnetic sensors require a magnet attached to the wheel for speed and to the crank for cadence to trigger a sensor. This can be tricky to align in such a way that's consistent and reliable. The non-magnetic sensor uses accelerometers to determine the rotation of the wheel or crank so no magnets!