Maybe I can help out a bit with some personal experience. I spent a couple of days researching as well as trying out a couple of the popular devices. My goal was to find a activity tracker that can somewhat accurately track and RECORD heart rate AT ALL TIME.
Now the reason why I had cap the "record" and "at all time" is that I noticed when it comes to heart rate monitor (and actually continuously recording it), there are two types of devices that you can buy.
1. This kind of heart rate tracker, would display your heart rate as you are wearing the device. However, they DOT NOT record your heart rate at all time. They only engage and starts to record your heart rate when you manually click on the "go" and tell the device that you are in exercise mode. They are also not meant to be a all day long device, I believe they last 8-10 hours at most. Devices such as Rhythm+, and Mio Link class heart rate tracker.
2. The other kind of heart rate tracker is designed to monitor and record your heart rate at all time regardless if you are exercising or not. That means if you are sleeping, exercise, walking, or simply just staying alive, it will keep track of your heart rate. Of course they also offer the ability for you to manually tell the device when you are ready to exercise. For example, Microsoft band, will record your heart rate at all time, and you can also tell the device when you are ready to exercise or go for a run. Other similar devices are Basis Peak, Fibit Charge HR.
(Note, I have yet to come across any all day heart rate monitor/record device that will link to the iPhone health app heart rate section yet)
My issues with the Microsoft band is that it is big (but not bad), however the horizontal screen means you have to wear the actual device at the bottom of your wrist for ease of reading, and since the screen is made out of cheap plastic, I have already received crazy amount of scratches by the end of the first day. Another issues is that despite the many sensors the MS band offers, the app is not as good as it can be. I.e., if you want to look at your heart rate after a whole day of record, it will only show you the highest heart rate of the day, you can't drag around difference time to see what your heart rate is at during a specific time. I personally think the MS band is a decent device, but the app can use a lot of improvement and please design something that I don't want to wear at the bottom of my wrist.
So with all that, I'm currently looking forward to the Fitbit Charge HR (for sure won't support Apple Health Kit), and not surprisingly the Apple Watch.
Anybody else have difference experience/device they like to share? Specifically with the heart rate stuff?