Of the 2 existential threats there is disagreement which would be more lethal, mirror life or nukes? One might take care of the other. Because the radiation and climate change, blocked sunlight, would likely do away with the counter spiral based life, that, history repeating, would escape from a Chinese laboratory. Name your poison, literally.
I'd still go with nukes, being an oldie, and considered the decades of experience we've invested in nuclear weapons. I'll give it hands over any bio threat, including an anti-chiral microbe. Would anti-life even survive in the open against the random actions of nature?