Both have their benefits. Some people believe that a Bluetooth connection is more secure than a Wi-Fi connection. And in the past, Wi-Fi was more accessible than Bluetooth.
But now, it is usually a matter of opinion. Though if your Wi-Fi cuts out, your scale wonâ€t be able to sync until it reconnects. While that is not an issue with Bluetooth as long as the Bluetooth is enabled on both the scale and your phone.