High difficulty for sasori.
sasori's only weakness is short-range but at the same time its an advantage because he could just lunge at u with the third's buzzsaw, poisonous gas, poison needles. or an iron sand jutsu.
kakuzu is fast yea and his only advantage is short range on sasori. sasori is almost dead by the time he wins. if kakuzu uses his iron skin, sasori's fire or water would just cut through. its not ordinary fire and water. they cancut through rock and metal.
plus if you look at there databook, sasori is 34.5. kakuzu is 32.
i feel sasori is being wayyy underrated cuz he was the first one dead. if you watch the episode sarori fought sakura and chiyo he dodged the monster looking thing by using his heart transfer jutsu. and that was pretty fast as it was floating full speed at sasori. kakuzu isnt as fast as a flying mouthmonster flying in mid air full speed in 1 straight direction. sasori has all the time to heart transfer. kakuzu wont have antidotes, its not like sasori wont get a hit on him.
Sasori takes this ====== but is in the same situation as chiyo and sakura. (used a core on a dead puppet to switch bodies)