I agree about the fact Garmin has SO, SO many more fitness metrics, but after a decade + of Garmin watches including the Fenix 5S I had to look in the mirror and be honest that I'm a 40+ casual runner who knocks out 3-5 mile slow runs and every once in a while when I get too fat I might train and do a half marathon but so slow all the 70+ year olds wearing massive costumes are passing me. I simply didn't use or didn't have a use for many of the metrics I could see. On the other hand I am a big Apple user and really missed the seamless integration of notifications and such that Apple does within its own ecosystem. Now watchOS 1.0 was terrible for fitness with not only no GPS on S0, but only a single metric per screen while working out, no maps of your runs, no 3rd party ability to use HR, no 3rd party to send any data to the activity rings and more. I couldn't do it, but now 3 years later with watchOS 4.0 and S3 its more than good enough for this fat, slow runner having fixed ALL of that. Sold my Fenix 5S, got a SBSS S3 and I'm not looking back!
I'm in the same boat as you, being 70, I've pretty much decided that the ultra's, and the Tour de France are out of my future, so while I have owned Many Garmins, from the 910XT, through the Fenix 3 series, (both HR, and non), Fenix 5, 5x, and a few in between, also Suunto's Ambit 2, and 3 Peak, and their Spartan line, the Apple watch gives me what I actually need to know, and does it efficiently. The other metrics, (I'm a data freak, been in computers for a LONG time), I can move to either Garmin Connect, or preferably, Suunto's Movescount. Does all I need, plus there's the added convenience of music on the run, notifications that work within the system, and the many other features that neither Garmin or Suunto have. Best all-round decision I've made. It should be said that I do not use the built in HRM on the Apple watch (Series 3, SS), but use a TickrX for actual classes, or gym work, but if I decide to take it easy, and do a simple mall walk, I have a Scosche Rhythm+. Although I'm thinking about buying the Wahoo Tickr Fit Optical HRM to replace the Scosche.
If I were 50 years younger, maybe the added cost of the Fenix 5x, or so, or the Suunto Spartan if they ever get it going correctly, would be in order, but for now, AW wins.