H10 uses better electrodes for heart rate measurements. It also includes dual bluetooth/ant+ wireless connectivity so you can sync to two devices at the same time. It also has internal memory to record your heart rate measurements without wearing your smart watch. The strap is also made from nicer material too. The H9 models have the same accuracy to the H10 but its just more feature focused instead.
had a treadmill that measured my heart rate at 240 bpm during a warm up while walking. It started to flash emergency stop on the console screen. Scared to death for my treadmill. Grip sensors on cheap treadmills arent very reliable.