If i remember correctly, those coatings dont rust, which looks nicer. However i've heard of stories about the powdercoating. Because some powder coat the entire rotor which, when you apply the brakes goes all over your rims and brake pads. Im not sure about any disadvantages to the nickel plating but im sure they are probably out there.
nickel plating is just for looks, I got the 13" cobra kit a few years ago, the center cap rusts right away. but i dont care. brakes are for stopping the car. not for bling bling. imho