Thanks for all the replies.
It seems the Scosche is worth trying then.
But what about other watches? I like the idea of having one device rather than an external HR sensor be it chest strap or something like the Scosche on the arm.
Is there a difference in the HR sensor design between the Fenix and the 935 so it would be worth trying that one instead?
One of the advantages of the Scosche is the ability to position it where it works best. A watch won't allow that.
I've had a Scosche Rhythm+ for 3 years. I wasn't overly impressed for the first few weeks I had it due to getting some erroneous readings and a few dropouts. However, when I moved it from the recommended location just below the elbow to the front of my upper arm just above the bicep, it became highly reliable. I've used it in that position ever since and it's been pretty bulletproof. Initially I used it for runs and stuck with the chest strap for the bike, but soon enough I was letting the chest strap gather dust. The data is fairly indistinguishable between the two. The optical has a bit more lag to changes in HR but it's not dramatic and it's also not really important for endurance training.
The Scosche Rhythm+ has 2 minor problems and that's about it:
- Battery life is only something like 7hrs
- Velcro strap can be annoying under clothing
The newer Scosche 24 version addresses both of these and adds a lot of new functionality such as built in data recording but is also slightly more expensive and seems to have had some quality issues on early examples (might be sorted out now). I wouldn't go back to a chest strap, especially for running. I just find them too uncomfortable. A watch alone would be great if it worked well, but it seems they need to be very tight on the wrist to work well and many have issues even then.