I had the 520 - sold it and got the Bolt.
The bolt is streets ahead in functionality for me. The ability to set up screens from the phone app and the automatic updates via bluetooth are seamless (although I sometimes get duplicate workouts to training peaks)
The screen is not colour but to be honest I find that better and easier to see - battery life is better as a result.
Also - as anyone who has accidentally deleted a ride instead of saving it on a Garmin unit can will know, it can be quite awkward. Its practically impossible to do it with a wahoo #strava4life