Plastic, easy. Even if you can kill it with your strike ball, why not kill it with your spare ball and take any chances of reaction out of the equation. Maybe you flub the release. Plastic's still gonna go straight. Plastic is cheap, why expose your $200+ strike ball to the lane, pinsetter, and ball return any more than you have to? Plastic also gives you another reaction in the unlikely event that there isn't enough oil for anything else.
SH