People really should at least TRY to justify their claims. Here's my attempt:
In close combat, Kakuzu is at a disadvantage at this stage in the series. His Domu is pretty much nullified by Raikunais, or even quickly activated regular raikiri's if Kakashi is ever captured by the threads. Sharingan grants precog and eye-contact can lead to genjutsu, so even with Kakuzu's vast experience advantage, he would be on the defensive in this situation.
Kakashi IS a ninjutsu-specialist, and even if Kakuzu far outclipses him in terms of technique-scale and stamina, he KNOWS EXACTLY WHAT KAKUZU CAN do. This makes countering/dodging a LOT easier, even if he can't match Kakuzu technique for technique. However, with five separate threats spewing elemental attacks or threads or punches from every direction, Kakashi would have to use a LOT of stamina to keep himself alive.
Tactically, they're about equal. Kakuzu actually outmaneuvered Kakashi in their first engagement, but that was when his abilities were still mysterious. Kakashi has advanced since then, and Kakuzu hasn't.
Speed won't really be a deciding factor.
Genjutsu is in Kakashi's favor, but he doesn't use it often. It MIGHT cause Kakuzu to lose focus for a moment, and that CAN be critical. But this is probably not decisive.
Stamina goes to Kakuzu. He hasn't ever really gotten tired.
Honestly, Kakuzu takes the first scenario about 7 out of 10 times, I think. Number 2 is closer to 50-50.
