Personally like the BW more than the M6.
That said; Here would be my tier list (in my personal ownership experience w/modern high gain amps) for drop C (bridge pickups) YMMV
1. Nazgul, Atlas, Mercenary, Black Heaven
2. Ragnarok, BW, Duncan Solar
3. M6, Invader
If you want something different than the Nazgul, would recommend the Atlas for slightly cleaner and slightly more highs or the Mercenary for sheer push.
Both are about or slightly above on par in articulation with the Nazgul and low tunings, imo. I would slightly lean toward the Black Heaven over the Atlas if $ is no concern. Only very slightly. The Black Winter is amazing, but it starts to get pushed out of the competition (imo) once the tunings start dropping.
GL on your quest!