I believe Madara has explained this before...
The sharingan, in the end, is only jsut a tool that a shinobi uses.
how it is used becomes more important then the power of the tool itself.
Danzo, being a person who did not truly possess the power of the sharingan or wood element, would not be able to use the full extent of the capacity of those tools.
on the other hand, Itachi was the genius of the uchiha clan who has mastered the sharingan to such a degree that he is feared by quite a number of people.
and i would just like to point out this. if danzo could kill itachi, why would he fear him before and allow sasuke to live in the first place?