Oh obsidian mind is great, but it's a Warlock piece, and you described Warlocks with derision as a mere support class. You can't get the full benefit of obsidian when you have time delayed waves of enemies as in the hypogeum strike. Its great in crotas end too, but just as great as sunsinger because you have less chance of hurting yourself. They are both great choices, but still both give a decisively offensive role for warlocks, which is indispensable for any fireteam.