Fēnix 7 Pro Solar Sapphire. I don't know how it compares to Apple, but I love it. Very long battery life (well over a week) and it has sufficient sensors for me. I get blood oxygen, heart monitor (pulse, HRV), altitude, extremely good GPS, message notification mirroring from my phone with canned responses, touchscreen and buttons. The "proper" LED light is more useful than I thought it would be. Garmin Pay only works with Revolut in my country, but at least it's usable - so do your homework here. It can track lots of sports I don't do, I only just it for cycling, running and swimming.
Aside from raw sensor metrics, as someone else said, their Body Battery is pretty decent. The biggest thing for me was seeing a long tail chart for things like sleep score, HRV, average resting heart rate etc and noticing sharp differences on days that I was either sick or drinking alcohol. The effects of a couple of beers on me was similar to COVID (except that they only lasted about 24 hours). But these are things you'll get with any device with long enough battery life that you'll wear it "all the time"
I switched from a Garmin Instinct (several years ago) after having used Garmin GPS devices and watches while in the military to an Apple Watch Ultra now. The quantified scientist channel on youtube had some views that helped me decide (https://www.youtube.com/watch?v=YVhmzxpw5Gg).
Hardware-wise they're largely similar, with some additional edge use cases (depending on who you are ofc) for fitness and outdoors with Garmin.
On the Garmin side for software, one thing I really appreciated on Garmin was the "body battery", or estimate of your energy, as well as health trends, etc. Apple has caught up with many of the features in the last year, and has a little ways to go. Apps like Athlytic do a good job filling the gap, but Apple really is making that gap quite small. Garmin locks your workout data down to its garden somewhat limiting your exports and has it more readily cloud accessible than I'd prefer; I felt the Apple privacy protections were better after reading the TOCs and looking at how the data is stored on each service.
What ultimately made me decide to go with Apple Watch is the above data consideration, the smooth functionality and integration with my iPhone, and the rapid pace of development Apple has had. Also, the cellular, texting (not saying it's the preferred method), Apple pay, etc. are all great to have on the Apple Watch and I can go for a run with only that.
After I paid more attention to my body metrics, I also changed my habits and started focusing on hydration more, when I drank alcohol and how much, and I was able to make good choices that help me feel much better to tackle the next day.
The accuracy isn’t as important as the trend. Don’t expect hospital grade ecg results but either can certainly tell you if things are better or worse since you’re wearing the same sensor.
I’ve got the AW Ultra and I’m quite fond of it, ability to use it without carrying my phone (even audible audiobooks) is the killer for me.
I have a Garmin Epix Pro. I highly recommend the Epix/Fenix line, and these measurements / metrics are far superior to what you get with an Apple Watch (my wife has an Ultra). The differences become even starker if you are an avid athlete and use your Garmin to track your workouts/performances.
Trying to decide which one to get.