i don't examine too many spark plugs from gasoline-fueled, light duty engines. but i do see spark plugs from negative-ground and positive-ground ignition systems using a coil per spark plug. most of the electrode erosion is always on the center electrode. and there is a reason: it operates at higher temperatures than the ground electrode...more of the center electrode material is vaporized with the arc because of its higher sustained temperatures.

in my experience, platinum electrode spark plugs are a marketing gimmick.