Which unit is better?
I know they both offer the same thing pretty much... so perhaps looking for an answer who owns both.
The main thing is the performance... lag-free and doesn't crash or corrupt files.
For several years I used a Garmin 800, it was very poor. So poor in fact when I was selling it to a friend I told him everything I felt was wrong with, let him trial it for a month before he made a decision.
I haven't used the 520 in anger but did buy my son one several years ago, pre Wahoo, and I did like it's relative simplicity to the 800.
Without being able to offer a direct comparison I would say buy a Wahoo every time. The Wahoo's simplicity in set up, superb reliability in navigation, battery life and ease of use make it the easy winner over Garmin.
My feeling is Garmin have sat back and taken the money from cyclists for years without ever investing the profits in improved devices or software. I am far from being a software expert but the impression I've gained is Garmin initially probably had a decent device. With each upgrade, next generation, etc. more and more features have been piled on to the same basic software making it creak and groan and eventually very crash prone. My evidence for this is only that if I turned off all functions bar navigation my 800 worked well. Turn on the features I wanted to use and it would regularly crash.
The other aspect of the Wahoo which is vastly superior to Garmin is the use of an app for set up. It's simple and easy to use whereas with the Garmin I always had to get the manual out to make changes.
The Garmin 800 left me feeling this company has exploited it's position as market leader through constant product "upgrades" which it has failed to support with any radical software improvements. Ultimately I felt ripped off. I'd no longer consider any Garmin device
Wahoo on the other hand have had the advantage of starting from scratch and creating a product which does exactly what it says on the tin. It does worry me Wahoo seem to heading down the route of coloured maps etc. which are just so unecessary.