For those, like me, who don’t like using chest straps the Polar OH-1 optical armband has been great for me. It never misses a beat (pun intended). I have a couple of Polar chest straps too, but I prefer using the OH-1.
According to DCR’s review the Whoop doesn’t seem very accurate for training purposes, which fits my experience of other wrist based optical sensors. But I don’t have any personal experience of Whoop. That review put me off getting one for now. I like the concept though.