I have used the ER-X with the EdgeOS, so I can speak to that a bit... I use OpenWrt on different devices, never used it on the ER-X.
This is true, but just for the record, it is a linux based OS at the core, but the router parts are based on the Vyatta OS.
EdgeOS is generally updated reasonably frequently although it has been a while since the last update was released (mid March 2020, so it's been 6 months as of this writing). Version 1.10.11 is believed to be the last update to the 1.x range. Version 2.x is the future path.
1.10.x was rock solid. 2.0.x < 2.0.8 was extremely buggy and had massive regressions over the 1.10.x versions, but 2.0.8+ has been pretty good. There are some specific/obscure features which may not work as expected on v2 firmware at the moment, and some people who have very complex/obscure configurations have had some stability issues. I personally never had an issue with v2.0.8.
Generally, the performance of the device on EdgeOS is very good. You can get gigabit routing with the hardware offload engine enabled. That said, QoS and Netflow are not compatible with hardware offloading, will silently disable the HW offload engine causing speeds to be capped ~250Mbps.
A known issue of 2.0.x is that there is a minor performance regression relative to 1.x firmware. This affects the maximum routing speeds, but is only really an issue if you are running gigabit service... it is a few percent drop in peak performance.
Configuration and UX/UI
The Edgerouter line is generally designed for use as an enterprise level system. The capabilities are pretty impressive and the configuration is very flexible. It natively handles features like multi-WAN load balancing or failover and multiple public IP addresses, among other things. The firewall is extremely flexible and can be configured with a high degree of granularity.
A key thing to know is that some of the operational models are very different or even opposite those in OpenWrt. For example, take the firewall. It is not a zone based firewall (although you can use ZBF but not in the web UI)... the model is essentially accept all except where explicitly prohibited. So inter-VLAN routing is open and works by default until you create rules to limit/restrict it. OpenWrt is the opposite using ZBF and you must specifically allow inter-VLAN routing.
The interface isn't particularly attractive, but it is not that bad. It is much harder to navigate than OpenWrt. It is not mobile friendly (at all). The command line configuration requires a lot more knowledge than OpenWrt's UCI system.
The default state of the EdgeOS has all the core features you'd need/expect. VPN support is there, but it is limited. EdgeOS can be extended using Debian packages. It isn't well supported, and you're kinda on your own, and most things you might add would require separate under-the-hood (CLI) configurations. The web interface is not as extensible as OpenWrt's LuCI environment.
OpenWrt's package options and management system is really good... way better than EdgeOS. And you can often install LuCI support for various packages in OpenWRT, making the UI so much more flexible than EdgeOS.
I like EdgeOS and I have used it comfortably for years. I also love OpenWrt and I use it on different devices. I cannot speak to the performance of the EdgeRouter using OpenWrt...
If you're using the device as a pure router with few or no added packages, I'd suggest giving EdgeOS a try -- it is actually very good once you learn how to use it, and it is nice to know another router 'language.' But if you want to do more stuff with packages, try out OpenWrt on the device and see how it performs. I'm guessing you already know OpenWrt, so that part should not be a mystery.
P.S. Hopefully others who are using OpenWrt on the ER-X can comment on the performance aspects.