It's been a game of leapfrog as far as I can make out. I have a garmin 1000, which was an expensive bit of kit but it has been problematic all along, including leaking in wet weather. Wahoo came along and stole Garmin's lunch, a lot of people I talk to on long-distance rides use them and really like them. How they handle mapping and routing isn't as good apparently, but they are more reliable.
That was a wakeup call for Garmin and apparently the new 830 / 530 are a big improvement and better than Wahoo - which doesn't seem to have moved on much since the Bolt was introduced.
So - without having tried either, but having read a lot and spoken to a lot of people, I'd say the Bolt is better than the 520, but the 530 betters them both.