Cross Chop over Hammer Arm. Electivire doesn't have very good defences, so lowering its speed would be a bad idea. Hammer Arm is usually used by Pokemon that are already slow, like Metagross, Emboar and Rhyperior.
Usually, you will have EV investment in Electivire's speed, and while Hammer Arm has a higher accuracy than Cross Chop, you'd be negating that EV investment. Cross Chop also has an added higher crit ratio.