To choose an Apple watch Ultra, a Polar H10 chest strap or a Cyton 3 lead ECG device with HeartPy if accuracy matters?

If we can by for the same prize of an Apple watch watch a Cyton board on Ebay and build from it with HeartPy 3 lead devices, devices which are known to very accurate, who is longer interested if the Apple watch Ultra plus is more accurate than the Apple watch Ultra? Do we need …