If you’re a runner or you spend a lot of time at the gym – go for the Forerunner 645. It’s a much better watch for people who focus primarily on running, and you can still track cycling, pool swimming and a bunch of different cardio modes at the gym.
The addition of music functionality and payments from the watch mean the Forerunner 645 has similar technology to that found in the Garmin Fenix 5 Plus, which is basically double the price.
If you’re a triathlete – Go for the Forerunner 735XT. Open water swimming and triathlon mode are absolutely essential for triathletes.
On the flipside, if you aren’t a triathlete, the Forerunner 645 is definitely the better option.