I would go for the Life Orb, personally. The 69% increase in power (30% Sheer Power + Life Orb boost) is EXTRAORDINARY. Nidoking is gonna be outspeeded anyway by most other Choice users. EV training a Nidoking for Speed and Attack with a Jolly Nature along with a Life Orb + Sheer Power could make a deadly combo. (not the best sweeper but could still be very effective with enough Defense to hold up)
Hope this helps!