Its good to point out the the advantages both have.

Kakuzu has super armour and charka scaling

Kushina has immunity and on hit scaling

In theroy it would require Kushina to take damage 25 times to reach the buff level of Kakuzu at 100 charka. As both are all rounders the stronger in a 1v1 is Kakuzu since he can be front loaded. Kushina has better sustain but lacks the ability to actually harm Kakuzu early on. Kakuzu biggest advantage is his mystery can have 0 CD and 2 standard attacks per round. Imagine if Itachi susanoo had 2 standard attacks by default.