Linux – Using eth0 and ppp interfaces in an interchangeable mode

I second womble's assertion that wpa_supplicant is the wrong place to do this. You want to set something up in the /etc/network/ subtree, perhaps as womble suggests by increasing the metric of the wifi route, or perhaps by some other means involving a custom script in /etc/network/if-up.d/ that would take the wifi interface down if the ethernet interface came up.

Info on making scripts for /etc/network/if-up.d/ is on the interfaces man page, under the 'IFACE OPTIONS' section.

Do you have default gateways for both interfaces? If the answer is yes then disable default gateway in the configuration for eth0.