Opinion
The fitting on a garden hose screws on and seals with a rubber gasket. When removed and a hose is connected water is forced in under pressure, and if there is a slight leak, no biggie. The motor is at idle or close to it. Once flushing is done a cap is placed on the fitting and similar seal is back in place. When running at higher RPM under load there is only a slight chance of an air leak.
A quick connect uses a O ring to seal and if the clamp and seal fittings correctly then there may be a slight leak but at about idle RPM all is good. The issue may come in at higher RPM's and if a air leak starts. The quick connects I've used are good but would not count on them to seal very well.
If it was me, I would use a quick connect, but also use a ball valve between the quick connect and the motor. The ball valve will seal
