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from my perspective of the two paints, black onyx resembles that of the black onyx stone (go figure with the name xD). it's slightly lighter than obsidian which results in a slightly less reflective property compared to obsidian. the difference is minute and you won't notice it unless you intentionally compare it side by side.
You can't even tell a difference. I have an 06 Black Onyx and just ordered and installed the obsidian GFX front, side, rear, and spoiler, you can't even tell they are two different colors.
Yeah, I think you'll only be able to notice the difference in the two colors if you were going to replace a door, hood, or something major. As for roof spoiler, I think you'll be okay.