Speed over water is what you want. Speed over land is irrelevant. This is why paddlewheel is superior. If you ride in a lake with no current, then GPS simplifies this as there is no current when the water is flat. No need to calibrate GPS. A paddlewheel must be calibrated when there is no current with a stopwatch over a known distance (such as a slalom course) or with a GPS, then is accurate.
If you ride where there is a current, such as the river I ride which is anywhere from 0-7 MPH either direction throughout the day and constantly changing, then paddlewheel is the way to go. If you ride on an inland lake with no current, you may prefer GPS so you don't have to calibrate the paddlewheel from time to time.