Black Oxidized, Ni Black Plating and Zn Black Plating

That would most likely depend on the environmental influences that will cause corrosion. One may work well with a certain chemical, while the other wont. You should consider all environmental concerns, and check compatibility of both to make your decision... If it's strictly cosmetic, have a few samples made of each, and do some real world testing to help make your decision, and always, cost is a contributing factor. (I'm guessing the Zinc coating is cheaper than the nickel.)