Should I get a Bluetooth or a WiFi OBD2 adapter?

For most people, a Bluetooth Low Energy (BLE) adapter is the easier choice. You connect it from inside Odo, with no pairing in iOS Settings. It doesn't take over your phone's WiFi or internet, and after the first connection Odo reconnects to it automatically on every drive. WiFi adapters work too, but you have to join the adapter's WiFi network in iOS Settings first, and iOS may not always rejoin it on its own.