I'd prefer an always on HRM that in particular records periods of irregularities. We were early fitbit users, however after they died, we replaced functionality with the mobile's functionality - when it is on your body!
Very recently have added a Wahoo HRM chest band coupled to the mobile/bike app. It & other accurate HRM's can monitor heart rate variability (HRV). Have not yet delved to see if that is really useful, however I purchased the Wahoo based on it being accurate enough for the job.
The Kardia device is mentioned above. The electrocardiologist expressed delight when I presented the Kardia results after not managing to get a useful Holtor monitor trace. The one issue I had with the Kardia was dry skin. Really helped to wet, brine, or gel your fingers. Also, although I haven't had an issue with the 5 androids & an ipad I've tried it with, I read it will not work at all with some mobile phones.