Should I use Wi-Fi and Ethernet at the same time? I'm on macOS Mojave, and was wondering if I should have Wi-Fi and Ethernet on at the same? Are there advantages to keeping on at the same time, or will doing something like this cause any problems or interferences? Thanks for any information on this!
There shouldn’t be a problem with keeping both on at all times. Your machine will choose one automatically and everything will work. You can also increase the priority of one system over the other so that your chosen system will be used if available.

All things being equal Ethernet is sometimes faster due to WiFi devices slowing down when lots of devices and access points are close together (like in apartments). If you need gigabit network speeds then Ethernet will usually be faster as you generally need to be close to your access point for gigabit WiFi speeds.

AirDrop requires WiFi as it directly connects two devices. Other than AirDrop needing WiFi and the generally better speeds with less interference on Ethernet there is no reason for preferring one over the other.