Which one hr monitor (wrist or chest) when you start an activity ?

If you're wearing your chest strap and it's paired to the watch, it will take over HR once an activity is started. Mine gives me an alert as soon as I hit start on an activity that the external HR strap is in use.

If you're wearing your chest strap and it's paired to the watch, it will take over HR once an activity is started. Mine gives me an alert as soon as I hit start on an activity that the external HR strap is in use.

Thank you for your comment. What kind of alert did you got ? I don't remember that my watch alert me on anything, just saw solid heart icon on watch. Nothing esle...I Will try to be more aware for my next run.

The alert happens within 3 seconds of selecting an activity, without going upstairs and grabbing my strap and trying it out I believe there is an audible beep followed by a notification saying something to the effect of "External HR Connected". If I didn't moisten the little receptors on the HR strap before I put it on, it doesn't start picking up HR until maybe 30 seconds into my run at which point I then get the notification.
For what it's worth it's the same notification you get when any external source is connected such as a BT footpod or speed/cadence sensor.

On a related, but slightly different topic... How do I know which HRM (optical or strap) ended up recording my activity. I get the "Ext. HR connected" notification at the start of an activity as well when I use the strap, however how do I know the strap ended up recording the entire activity and did not fail & drop out at some point?

I know it is paranoid of me, but I'm just interested...

One way to tell is whether or not you have the other advanced metrics the entire time such as ground contact time, cadence, etc. Those only come with use of the external HR strap. Also, in my experience the two HR readouts/data look wildly different. THE HR data from an activity tracked solely with the wrist based HR tracker is all but useless, all over the place and hardly accurate. The data from the chest strap is consistent and smooth and much more accurate.

One way to tell is whether or not you have the other advanced metrics the entire time such as ground contact time, cadence, etc. Those only come with use of the external HR strap. Also, in my experience the two HR readouts/data look wildly different. THE HR data from an activity tracked solely with the wrist based HR tracker is all but useless, all over the place and hardly accurate. The data from the chest strap is consistent and smooth and much more accurate.

Great point re: advanced run metrics - thanks. Will definitely use that as a guide.

Regarding OHR - this is my first ever OHR device and so far I've found running OHR to be in the ballpark for easy/recovery runs or hill/stair reps - i.e. efforts where fast HR updates & precise accuracy are not critical. Yes OHR is out a bit vs. the strap (I can tell by how I'm feeling/breathing vs. OHR readout) but it is acceptable for the run types I mentioned above.

For any serious aerobic/tempo or anaerobic sessions or for long runs strap is the only way to go.