Going with Kakashi mid/high diff.
While Darui has impressive kenjutsu, Kakashi simply outclassed Zabuza in their latest encounter. Darui beat Suigetsu, but it was in a large part due to elemental advantage and Kisame himself said he wasn't nearly as capable as Zabuza.
Kakashi has already outmaneuvered several top-tier ninja with clone feints, Darui would quite probably fall for that. If not, doton, suiton, and maybe even high-class raiton counters Darui's ninjutsu offense (could Kakashi block Ranton with dual Raikiri + Sharingan precog?) and a number of Kakashi's latest techniques are pretty deadly. Raikunai is nearly unblockable, Raiden has a wide range + high speed, and the Lightning Hound homes in on its target.
Taijutsu is in Kakashi's favor.
Genjutsu is in Kakashi's favor, he has incapacitated multiple ANBU simultaneously before. Even Darui's partner, C, fell for Sharingan genjutsu.
Stamina-wise, they seem about equal.