I've compared both directly against each other, and I ended up keeping the Grey Stache. Through my rig and bass, they sounded very similar but the GS had more grind in the mids and sounded slightly more gutteral than the BB, which was just smoother (probably due to the excessive gain.) But then my guitarist tried the BB and ended up buying it off me, so if you heard my band you'd hear GS on bass and BB through guitar 100% of the time. Both are gnarly and the best way to tell which you prefer is to try them both at the same time, as they can definitely sound similar and itd be hard to try one, then try the other months later and compare them.
I will say that if I'd never actually bought one and played it myself, I'd always be wondering "what if??" So def try both!
(I know you didnt want other suggestions, but when I had the GS and BB at the same time, I'd wished I'd also bought a Supercollider to compare all 3... its supposed to be similar to both the BB and GS)