The Beryl AX is a newer and more advanced router compared to the Shadow (GL-AR300M). 

Some key differences:

The Beryl AX supports the latest Wi-Fi 6 (802.11ax) standard, the Shadow is limited to older Wi-Fi 5 (802.11ac), so the Beryl AX can provide faster wireless speeds and better performance, especially in congested environments.

The Beryl AX also has a more powerful processor, with a dual-core 1.3 GHz CPU, compared to the older single-channel 2.4 GHz Wi-Fi N connection in the Shadow, so the Beryl AX can handle VPN and network traffic better.

The Beryl AX has more Ethernet ports, with 1 WAN and 1 LAN, compared to the single Ethernet port on the Shadow.

The Beryl is a little more expensive, but IMO well worth it.