:shrug:
very good point, but in my opinion i see them as equals because it states
"Like many members of his clan, Shisui was a wielder of the Sharingan. He was praised alongside Itachi for having the greatest mastery of and sheer power in its usage." and also
"Throughout his lifetime he influenced Itachi greatly as a friend, teaching him self-sacrifice and telling him that a true shinobi was one who protected the village from within its own shadow. This possibly influenced Itachi in his future actions." so from those two facts i formulate them being as equals because they were both praised as equals even though we have not seen him fight i can speclate that they are on the same level with itachi being more powerful with his two tools.

but we are all entitled to our opinions
In my opinion i dont think itachi would let someone weaker than him influence him, you would either have to be equals or stronger than or i maybe wrong any one has any input