you remind of the famous amigos4 of this website.
I think the phrase is "difference of opinion".
Ex. : "You may be right about the world economic situation, but I have a difference of opinion in the matter, there would be good days again."
2) "Sir, I beg your pardon, but I have a different opinion in the matter."
Though depending on the situation, difference in opinion may be used :
ex. 1) The difference in opinion on the issue led to voting in the parliament.
P.S. : I am not a teacher of English. Just someone who loves the language.
Student or Learner