In reply to Dark-Cloud:
These kind of threads tend to just get people recommending what they've got. So in that spirit...
I've had the Fenix 5 since August. Before that I had the Fenix2 for about 3 years and the FR405 for about 4 years before that so I've only had experience of Garmin.
On your points:
Battery Life
I did the CCC with my Fenix 5 - I had it recording HR via wrist, on full GPS with 1 second sampling and I got to the over halfway point at Champex-Lac with 50% battery. There was a Garmin stand there so I took the opportunity to charge it up for 15 minutes and I finished the race after 22 hours with about 20% still left - so I reckon I might just have managed it if I'd not recharged but it was good to have the piece of mind.
I haven't tried UltraTrac in anger -it does GPS sampling less frequently and uses the gyroscope to fill in the gaps and increase accuracy. I tested it at home on a regular run loop of 8km and it managed to create an extra phantom KM. I haven't done it enough to know if its a regular issue.
The 5s and 5X don't have as good battery life.
Chest Strap
Like you, I thought saying goodbye to the chest strap would be great, and during races and especially ultras I don't use it. However, optical HR is nowhere near as accurate as chest straps and if you want to use a lot of the running metrics like VO2 Max, training load etc then you need to continue using a chest strap for some of your runs.
I think this is an inherent flaw in the technology as opposed to a particular problem with Garmin.
You mention however you want to train to HR, and there's a lot of theory now in coaching that its not the best idea since HR is really just a measurement of your body's reaction to the work being done, rather than an indication of the level of effort (for example like a cycling power meter) and therefore lots of different factors (fatigue, altitude, temperature etc) can give different HRs for the same level of effort. For that reason try looking up Rate of Perceived Exertion (RPE) and use that as a better methodology*
The optical HR works well for all-day activity tracking and managing data like your average resting HR throughout the week so you can see if you're getting fatigued and overtraining.
Overall
All in all I'm happy with it - it syncs automatically with the app after a run which loads everything straight to Garmin Connect and also Strava. The watch looks good and works well as an all-day device although I turned off the notifications as the novelty wore off. The UI is intuitive and it feels comfortable. I also like the quick release for the strap so I can use one strap for running and getting sweaty, and another for everyday use.
*obviously you're free to do what you like and apologies if I'm telling you what you already know