Is the HR sensor of Fenix 6 better than of Forerunner 245

I believe its the same HR sensor, but there will be different firmware underneath.

A middle ground update for you, which would significantly improve your 245 is to buy a chest strap. Personally, their accuracy is miles above any hr sensor and its a cheap upgrade... plus its one that would work with a Fenix as well. If I was you, I'd try getting a cheap HR strap for your runs, see how you feel about the 245 with a better sensor for runs etc - then decide.

I'm after this test: I got a HRM-SS form a friend and compared it with the FR245's built-in optical sensor using the DataField called "Auxiliary Heart Rate FR945". Maybe it's just a problem with the specific strap (it's pretty old) but both (strap and watch) disappointed me on different issues:
- strap: even though I wetted it with tap water before each run, it usually was useless in the first 10-20 minutes.
- watch: on interval runs it's lagging so much on short intervals, that I (would) have no idea what is my maximum HR (if I didn't see it from the HRM-SS [assuming it's close to the real value...])

But even if the strap was super accurate all the time, it's not a comfortable option to me.

Where can I read about the firmware difference regarding the HR sensor? I guess it's the thing that some people in the FR245 forum complained about, that 245 doesn't get some update that other watches did get?

Ah, BTW: the 2 options I thought about are:
1. buy a Fenix 6 for me, and give my FR245 to my son

2. buy a FR55 to my son
Although the 2nd option is slightly cheaper (usually much cheaper, but because of the discounts it not much now), the 1st option would give me the barometric altimeter, and a bigger screen and Garmin Pay, and give my son some features that the FR55 doesn't have (even though the HR sensor on the FR55 is a newer generation I believe ;)

I would suggest its probably the age of the strap. I never run without a strap and mine behaves really well - the accuracy is very much worth it. Yhe HRM-SS is the removable one, so perhaps pick up a $10 replacement strap for the sensor and try again? 

Using the data field to compare probably added a lot of computation that might have caused lag - I'd retry the intervals test without the data field and look at the hr compared to an older, similar activity

Im not sure where you'd find the info regarding firmware differences, though I suspect the more grunt in the Fenix would make a difference

I doubt. We're talking about 30 or more seconds lag, not milliseconds... I also did the same intervals with a few days difference with only the watch and it measured more or less the same (~140 BPM after a 150m uphill full effort run) while the HRM-SS measured around 170 and that feels correct.

Strange... like I said, once I started using a HR strap I've never looked back as they're far more accurate and responsive to sudden HR changes, and also zero chance of cadence lock. That lag sounds like wrist HR lag for sudden changes,...

So you're saying Chest HR gave you the 170 and is more believable? I'd look at perhaps washing the fabric strap, because I, and many others, don't see that 10-20 minute issue and it does sort of sound like the fabric side of it is on the way out. Once the fabric starts to misbehave, its often better to buy a new fabric one. My current HRM-RUN is two years old now and still behaving, although I do have an old removable one as a backup just inc case

Here's a recent 10K, and note the only reason for the 1-min ramp up is I went from standing start to go...

The original strap at Garmin is $35 + shipping, almost the price of a new HRM Dual :(

I'll give a $13 Chinese band a try.

BTW I checked now, and indeed the band is dead, it's a wonder how it worked after 20 minutes (though in the last run I tried it didn't work at all... The next picture probably explains why)

Yeah, that's a little....road worn :)