Okay I just read some of Darui's abilities and that guy is very powerful. Though Kakashi is still better than him but it wouldn't be that much of a stomp. I'd give it to Kakashi mid-high difficulty.
Basically, Kakashi has the advantage of having the sharingan allowing him to predict and dodge Darui's attacks. From Darui's small skirmish with Sasuke, we learnt that his lightning and water combo is relatively ineffective against a lightning user. Darui still has that black panther but Kakashi can go underground to avoid it.
Kakashi can counter Darui's water with his own to avoid getting spilled with it. Kakashi is faster than Darui and better in taijutsu most likely. Darui is a kenjutsu master though and a lightning one too so I don't think Kakashi's main attack (raikiri) is a good idea to use against Darui.
In other words I vote Kakashi only because of his sharingan tracking abilities and the ability to go underground. A doton wall can also prevent him from getting hit by water.