There is more than one way to define intelligence and tbh the manga has given us this information.
Orochimaru;s intelligence lies in either developing technique's or creating them from scratch. Either kinjutsu or ninjutsu. What people dont understand is that orochimaru is the best, yes the best technique developer in the manga. Both hype and feats goes to him, something that people dont understand and give this to minato. Orochimaru perfected the edo tensei technique, it was incomplete when tobirama created it. Now some may argue that creating a technique from scratch is easier than completing a technique, however it differs. In certain scenario's completing a technique that was created by somebody else can be harder than starting from scratch. He also created the orochimaru no Juinjutsu, which has two variations which are the Ten no Juin and the Chi no Juin from juugo's clan's ability. He also created the Fushi Tensei which grants him a form of immortality and the Kawarimi no Jutsu though his form is different from the traditional method, in his version he sheds his skin, which has been noted to be near impossible to detect. Its a fact portrayed by the manga that orochimaru, due to his ideology and selfishness sits at the top of developing technique's, kinjutsu's.
This also says it all;
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But orochimaru's intelligence is in jutsu development and to an extent planning, however he lacks in battle intelligence and i can say he takes the title for the lowest intelligence in battle terms, though it can be argued that its a form of confidence/underestimation of his opponents. This form of intelligence is something that itachi excels at, along with planning. Given hype by every character that has come close to him and witnessed his battle intelligence to an extent that his recent opponent, kabuto heaped numerous praise on him.
Even within a clan of geniuses itachi's shines the brightest, and as the DB says he was the envy of the clan. He also was the one chosen within a clan of geniuses to become a spy, that says it all;
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To put it simple, their intelligence is different. Both however are masters of their respective intelligence category
(kinda like Kabuto sorta is--more or less.)
