I have the Garmin 920XT tri version. Similar but not as expensive or as bulky as the Fenix 3 which i found too big for everyday wear. I think for accuracy the strap HRM is better than the watch HRM particularly when wet, I have the vivo active but its wouldn't work with the Tri HRM so the change to the 920.
The Tri HRM stores the HR data when not communicating with the watch and uploads later. So good for when you fall in (which I do) and comes with a swimming strap as well that stores even more data. I also use when I box and just leave the watch running in my bag, then down load the HR data when I finish.
The advantage of the Fenix it has bigger screen and more interaction with your phone functions almost Apple watch like but not the same.
I wear it for everything and everyday and really happy with it buttttttttttttttttttttttttttttttt if I could find a suitable app I would be like PT and buy the apple 2 in a nanosecond!
I find this guy does the best reviews, long but worth reading
www.dcrainmaker.com/2014/11garmin-forerunner-
920xt-depth-review.html