Both men have one month prep but cannot spy on the other. They take the battle out in the open in the living world right in Times Square. Many civilians will be around the place. KO/BFR/Death all count, but must last for at least one month or be permanent. I'd say morals off, but neither really have morals.
Mayuri's unkillable (can literally have his torso shot out and completely destroyed and just turn to goop) and probably much faster than Sinister. He pokes him with his super slowdown/speed up drug and Sinister is locked in his own head for eternity.
@Ultimate_Riddler: Being faster, His zanpakto's shikai emits a poison made from his blood and there's his bankai
Doesn't Mr. Sinister possess telepathy? How would Mayuri fare against that?
@Ultimate_Riddler: I don't think Mr. Sinster would want to go in Mayuri's head and I think it would be about as effective on Mayuri as it would against Deadpool
Telepathy isn't always about going inside the person's head, and that's quite an assertion you're making comparing Kurotsuchi to Wade Wilson. Both men are twisted and sadistic to the point I doubt Essex will see anything in there that would even remotely unnerve him about the Soul Reaper. Regardless, the poison Shikai isn't really an obstacle Sinister would have to concern himself with. From Sinister's Comicvine bio: "--complete control of his body at the cellular level, allowing him to shape-shift, regenerate and be virtually immune to injuries. The character has exhibited the ability to teleport, though Beast believes this is accomplished through his tesseract headquarters."
Telepathy is founded on the basis of going inside someone's head or reading that person's thoughts in an attempt to predict one's movements and while Mayuri is twisted and sadistic, Wade is just plain crazy and probably suffering from various forms of dementia among other things. Anyway my point in comparing them was not in the sense that Essex would be disturbed if he went inside Mayuri's head but rather much like Wade, no one knows what exactly is going inside his head. Not i'm not exactly sure how Mayuri's poison works but it is said that he made it from his blood, only he and his daughter are immune to it, and that he constantly changes it in case someone becomes resistant or immune to it.
Now per OP I'm assuming that while they cannot spy on each other with the prep thy can still find a way to get knowledge on each other and as such knowing how Mayuri works he'd probably create a special poison that can affect on a molecular level and I think that Mayuri could also invent something in regards to Mr. Sinister's abilities but then this is just speculation on my part, I mean it's not far fetched or anything because he keeps breaking and remaking and improving upon hi Ashogi Jizo
Although I like the idea of this fight, putting sadistic against sadistic, but this is a stomp. Mr Sinister's strength lie in working behind the scenes and putting himself into a position to continue his research. Mayuri has a similar ambition, with that creepy anime twist to it, but he is also a good fighter. Given a month of prep I would be fairly certain that Mr Sinister could alter his genetics to be immune to any poison Mayuri could cook up, but when you factor in the speed and swordsman ship of Mayuri the poison would be just one of the many concerns Sinister would face. If they were given full access to their labs and henchmen Sinister may stand a chance simply due to being able to send clones of powerful mutants into battle but one on one I dont see how he would be able to stay in this fight.