How are saguaro cactus plants unique?

A saguaro cactus [Carnegiea gigantea] is unique in the towering heights and the advanced age that it reaches in a bright, drab, dry, extreme, harsh, hostile, monotonous environment. It's capable of reaching a mature height of 40 feet/12 meters, and living 150+ years.

But it actually shares that uniqueness with the old man cactus [Cephalocereus senilis]. The old man cactus likewise may mature to 50 feet/15 meters, and live for 200 years.