Sorry, I don’t know enough to describe the differences. I have not researched the EPR Design.
EPR still relies on active safety systems (pumps, motors, emergency diesels for AC power etc,) and relatively conventional construction practices while the AP1000 utilizes mostly passive systems -— gravity, pressure, convection etc and is a ‘modular design’ in that most of the plant systems will be factory built modules shipped by rail or barge and then assembled on site.