I have some first-hand experience with two devices. The Fenix 5 and the Moov Now.
Subjectively, I’d argue that the Fenix 5 is sleek and gives you enough features to satisfy the smart watch itch. It, like every other wrist watch does not gauge HR well if you intend to use it to gauge HR during interval training. For averages over longer time it does the trick and even had HRV support which works okay.
I say okay because the device is just continuing to gather data and compares back against previous data points. So, if one was to start using it during a training camp it’d be incapable of saying “you’re super stressed right now” because there’s no data to indicate this isn’t your norm.
I’d still have and use it if I didn’t have a hard time not to obsess about its activity level measure (possible to hide but hard to forget and I’m a techie so even if I have to jump through hoops anything less than a hard block doesn’t really work for me).
The Moov is a fantastic piece of kit but it’s a bit weird. It doesn’t have a display! But the battery is amazing. It has more sensors and can coach a lot of activities, for me the best thing is that it can coach my running gait. Can be paired with a chest strap to track HR. Will keep track of sleep and activity but you have to start the app to see it.