I had a Fenix 3 for about a year, in my experience it was wildly inaccurate - had me running in totally different street often and unreliable, new updates every week, where you pray the thing will survive. Randomly paired with other peoples HRM during races, then paused while it told you it was out of range. I really missed the 310XT, if you can cope without the phone syncing stick with it, one of the best watches I've owned.
I'm now on a Suunto, very happy, rock solid I've no idea how to switch it off let alone factory reset it once a week.