Also in Belgium, both devices are priced equally. It makes no sense to get the 500 if you ask me.
After some time using my Rider 550 I can't say it is bad overall. In fact it quite quick, responsive, intuitive and nice device.
The issues I'm not happy about are not device itself. It is Mydrive Android app which do not working as intended. Also response from TomTom support about their decision to plug off Tyre code for new Rider is some kind of totally nonsense (check their response in my post above).
About which one is better I can't answer that question easy. I did not had a chance to ride with Garmin. Just had 595 in my hands and wasn't really impressed about interface design. I have no idea how that one working on the road, so I can't recommend something I never used.