The two clubs are separated by only three points with three games to go in the season, meaning City - who have a superior goal difference - can leapfrog their rivals with victory.
The Red Devils may have a long list of titles, while City are the new kids on the block in terms of challenging for the Premiership, but Mancini told reporters ahead of the game, it would decide which club is the biggest in Manchester.
“If you win the title there can be no debate about who is the number one team in Manchester for this year at least. But United have a fantastic history and it’s impossible to change that,” he said.
He added that City were aiming to become one of Europe’s biggest clubs, hoping to emulate the feats of United and Spain’s giant pair.
“Our targets should be to match the achievements of United, Barcelona and Real Madrid,” Mancini said.
“I think that now we are set up to win many trophies in the future. People say it would be easier if your main title rivals were not from the same city. I don’t agree. I think it’s good to win the title against your neighbours, rather than a team from another part of the country.
“I had this in Milan with Inter. It’s more satisfying to beat a local rival and it’s important for Manchester to have two great teams.”
Meanwhile, United boss Sir Alex Ferguson said if City won the derby, he expects them to go on and win the title, despite having to still face a tough trip to fourth-placed Newcastle.
Ferguson said: “I think City probably would win it. Yes. I think, with two games left, they’d have a great chance.”