I’ll add that if you care about working out with a watch to track heart rate, the Apple Watch is pretty dismal. I moved to pairing mine with a Polar H10 strap so that I could get much faster feedback on heart rate recovery post sprint. In addition to using an optical sensor, the Apple Watch may also be doing something to smooth out the signal. I just know that the Polar seems much more responsive and matches how I am feeling.
Chest straps work with ECG, so they're hard to beat for heart rate tracking. That said, Apple watches are some of the best wrist wearables for heart rate tracking. Most other brands aren't as accurate and many don't come even close.
Check out the Quantified Scientist on YouTube, he does some interesting testing on health devices.
