I would strongly recommend an overflow vs simply using a PVC
pipe to syphon the water out of the tank. Overflows are fairly inexpensive and will go a long way in giving you pieace of mind over the PVC
syphon method. Should air get into the PVC
the syphon will be lost and the pump will continue to fill the tank. If the pump cuts off the pVC
will continue to syphon the water into the sump until air can get in to break the syphon and then when power returns the syphon wont start back up.
Overflows are designed so they will start their syphon up again when power fails and since they remove water from the surface of the tank they will only drain water as fast as the pump is filling the tank. Ideally you size your pump smaller than your overflow will process. This will asure that you dont overdrive the overflow and slowly overfill your tank.