Depending on the rules you have set for your MX64, the Meraki GO GX50 firewalls have limited firewall options compared to Meraki Enterprise hardware.
For example, the MX64 has options for layer 3/7 firewall rules as well as content filtering while the GX50 only has layer 3 rules. The GX50 is also limited to site-to-site VPN with other GX50s while the MX64 can be configured with non-Meraki peers.
Meraki GO equipment is managed through the Meraki GO mobile app or web app (https://web.meraki-go.com/) while the MX64 is managed through the Meraki Dashboard.