Both work from https://cncfoam.com; pick by convenience. USB-Serial is rock-solid and low-latency — plug the ESP32 in, pick the port in a Chromium browser (Chrome/Edge/Opera/Arc/Brave; Firefox/Safari lack WebSerial), and stream. It is the best choice for reliability and for first bring-up.
Wi-Fi (FluidNC's WebSocket) frees you from the cable and lets the machine live across the room, which is lovely once everything is dialled in, but it depends on a stable network. A common workflow is to develop and test over USB, then switch to Wi-Fi for everyday use. Either way you can also just download the G-code and load it from an SD card if you prefer offline.
Wi-Fi (FluidNC's WebSocket) frees you from the cable and lets the machine live across the room, which is lovely once everything is dialled in, but it depends on a stable network. A common workflow is to develop and test over USB, then switch to Wi-Fi for everyday use. Either way you can also just download the G-code and load it from an SD card if you prefer offline.