I`ve just been doing research and All I can find besides the fact that they look very much the same is the fact that Besides both of them having a black blotch on the cheek, near the tail and a large black line along the side of the boddy with a blotch in the middle, is that A. Metae seems to have more orange coloration mixed in around the black line, and Diadema has very little to no orange.
So based on that observation my guess would be Metae, but unless we can get an expert or ichthyologist who can count fin rays and scale lines in we may never know