Best Smartwatches for Triathlon/General Life

Looking to purchase a Smartwatch to help me with my training etc. I have a budget of £400 but don't necessarily want to spend that much. I've been looking at Garmin Venu, Forerunner 735XT and the Forerunner 935. I like the idea of being able to stream music from the watch, and it to be compatible with additional monitors, so it'll be future proved for when i eventually get one.