Polar H10 vs Garmin HRM-Dual, help me analyze which one to trust

I can say little about your problem, just this: the H10 was the first and youngest of my 3 chest straps that I threw the trash.

Constantly strange results, always battery empty.

Even if it is technically cool, for example, can supply several devices simultaneously, he is lousy in practice. If you write Polar support an email, you get "Undeliverable, because the email address does not exist".

I would test another device in your place, for example this one: https://amzn.to/3afYgAA

However, it also breaks down after 2 years and the distribution in Germany is strange when it comes to warranty claims (1 time the device had to be replaced, then it was pushed on the belt), so I would not buy it again, but it costs little for a comparison.

I am using daily either Polar H10 or HRM-pro. I have never had any issues with the Garmin strap. Sometimes I have wild readings at the beginning of a run or a ride with the H10, but it is because of dry conditions with light synthetic jerseys/t-shirt. If I am careful about wetting the strap and/or using some conductive gel, no problem at all.

It is pretty clear that the HRM-dual disconnects sometimes when you are at maximum effort or just at the end of a set, which could explain why it misses the top HR, and takes a few seconds to catch up when it reconnects.

I don't have this problem when I do strength training or HIIT with either strap.

Since you don't have this problem with other, more sweaty activities (you mention running intervals), my guess is that the strap simply loses (electrical) contact with the heart. There can be 2 reasons. The strap slips too much as you finish your set (for example you bend over, or you extend your arms up) and/or the strap is too dry. Have you tried applying conductive gel to the strap?

I suspect that your problem is low battery voltage in your HRM-Dual. The voltage drops during usage, and when it hits a critical value the device will reboot (explaining the dropouts). Notice how the dropouts tend to occur near a peak in heart rate, which might correspond to maximum current drain. During the reboot processes it is likely that the power drain is a bit lower than normal, so the voltage recovers above the critical value. The HRM then takes a short while to resynchronize to your heart rate (explaining the overshoot and the decline). I've seen a similar issue on my HRM-Pro, and it was fixed by changing the battery. On the HRM-Pro at least, the battery OK indicator is useless, and will say that the battery is OK even when it is dead flat. Take care when changing the battery to maintain a good seal with the o-ring, else water will leak in and damage the electronics.