Upgraded from a Garmin Fenix 2 to the Forerunner 935. What won me over was definitely the smaller form factor, added GLONASS, and the wrist based heart rate monitor.
In principle, the most important training metrics (pace/speed, distance, power, etc.) for swim, bike, run have stayed the same and there is no added benefit from upgrading.
The smart watch features are nice, but I don't really use them. [This review was collected as part of a promotion.]