|The big difference is that with an ATX power supply, mains current never leaves the power supply. All wires coming out of the power supply carry 12v at the most thus reducing the chances of an electric shock.|
With an AT power supply there are four thick wires coming out of the power supply carrying mains current the connected to the front panel switch. Any computer with an AT power supply is going to be at least ten years old and probably a lot older.
That is why with AT powered computers you always got the message, "It is now safe to turn of your computer." once the shut down procedure had completed.
With ATX power supplies the mains power is turned off electronically with just a small residual current going to the motherboard in order to turn back on again.,