My personal preference is to use the “different” port on a router for the “different” traffic; upstream connectivity on both devices in the port that happens to be labeled “WAN”.

With devices with multiple phys (eth0 and eth1, for example), then you would want to balance the load on the two phys. While I don’t have an AR750, I believe both it and the AR300M are single-phy devices. You can confirm in the network setup if there is only one phy in use, but with two VLANs, such as eth0.1 and eth0.2 configured for “WAN” and “LAN” connectivity.