How did we make the comparison?
I paired Heart Rate monitor, Magene HR64 Chest wrap to my Bike Computer XOSS G Plus / G+. The readings from Apple watch 6 went to my iphone (12).
For this test, one of team members cycled for 30 minutes and his heart rate fluctuated between 70–156 bpm from the Heart Rate Chest wrap sensor.
Whereas, in iPhone for the same ride, the heart rate fluctuated between 73 - 156 bpm. So in the both cases, the Avg Heart Rate is 134bpm
From this research, we are not making the point on which heart Rate monitor is better. But, when you are using Bike Computer with ANT+, it make sense to use ANT+ compatible sensors (be it Wrist band or Chest Wrap). So the readings are pushed to Bike Computer and aggregate with data from other ANT+ sensors (namely, Power meter and Cadence) for greater analysis.