My vote goes to Snakes. An almost identical matchup (Kemono-dono vs. Novel Kars) was judged overwhelmingly to result in Heydrich's victory (using IKaBey Reinhard), and IKaBey Reinhard is *far* weaker than Mercurius' Sensory (since in Kei's route, despite 4-C Reinhard seriously trying to kill him, Reinhard is unable to). But if we judge solely on Mercurius' merits, he should still be easily able to take this.
First of all, Kars' ability to copy anything would not work against Mercurius. Not only can Kars not properly perceive the source of Mercurius' abilities (and thus not understand them), since Kars isn't 1-A, but, like what was judged in Reinhard v. Kars, even if Kars *could* copy Mercurius' abilities, it wouldn't do him any good because Kars doesn't have the sheer volume of souls required to use them as well as Mercurius (in the case of Mercurius, that number is "every soul to have ever existed").
Secondly, Sensory Mercurius is capable of using Acta Est Fabula. From
Habaki's Page, it says that because Habaki is the Sensory of Hajun's Tumour, he's able to use the Tumor's Taikyoku, Shoko Mandala - Yaoyorozu. So if Mercurius actually fought seriously, he could annihilate Kars using, say, Sequere Naturaam, or to be extra thorough, Disce Libens, which not even Kars could escape unscathed, since it annihilates the entire universe.
Thirdly, Kars' most powerful attacks are wholly useless against Mercurius. D4C Ultimate doesn't work on Snakes because Snakes is present in every universe affected by the Throne (which is any universe where he is present, because even if the Sensory were to be brought to a universe outside the Throne's influence, the Sensory is basically a terminal for 1-A Mercurius, who would then incorporate said universe into the Throne, giving him an even greater powerup), so the best that D4C could do is escape to another universe where Snakes can't touch him, in which case Kars would be BFR'ing himself. Made in Heaven Ultimate Requiem is useless because Mercurius is immune to time-based attacks that aren't 1-A.
However, in character, it's unlikely that Kars could actually affect Snakes in such a way that it would prompt him to fight back or do aything, which is Kars' only saving grace - because it hasn't been specified that the two are bloodlusted. If they're both bloodlusted, Snakes instantly wins because his attack power is Outerversal, which not even Kars can sustain a hit from.