As a former Smokey Granite Mica owner I loved that color ...even though I hated the name. Nebula is too light for my tastes, especially when a co-worker parked his SGM 2IS next to the Nebula Gray loaner that Lexus gave me. Don't get me wrong, Nebula is a fantastic color but it wasn't my SGM I loved. So for me, if it was a choice of Nebula and Obsidian, definitely Obsidian. The car looks great in any color though.
I say pick based on how feel rather than what logic tells you because you will find a way to make anything work if you love it enough.