I tried a MIO Link wrist strap and a H7.

The results were markedly different. The latter had an old battery but that was probably ok and I continue to use the h7 daily.

the difference could just be that they were not taken at the same time.

the MIO is marketed as (I think) having 'medical' levels of accuracy

I presume generally it is best to stick to one device but having a wrist based device would be preferable to me as the early morning half -asleep process of putting it on would be MUCH easier.

however there would be little point in doing that if the reading was unreliable (MIO)

anyone else used a MIO Link?

is there a protocol for testing the device accuracy?

does it actually matter? so long as one device is continually used?

People have already tested the BioForce HRV system using a Polar H1, H6, and H7, The MIO Link, and the Armour39. What they found that although the Mio and the Armour were accurate enough for heart rate readings, they are not nearly as accurate for determining heart rate variability. I would personally recommend something from Polar or Cardiosport for the most accurate HRV readings.