Differing views, but the reality is that whoever wins the game at the Etihad Stadium on Monday will be favourites to be crowned champions with United three points clear at the top and three matches of the season left.
The battle for Champions League places involving Arsenal, Newcastle United, Tottenham Hotspur and Chelsea also becomes more intense after Chelsea reached next month's Champions League final.
Should Chelsea go on to win the Champions League, whoever finishes fourth will not qualify for the elite competition and go into the Europa League.
Meanwhile, at the bottom, the fight to avoid joining Wolverhampton Wanderers in next season's Championship, is just as frantic with Blackburn Rovers, Bolton Wanderers, Wigan Athletic, Queens Park Rangers and faltering Aston Villa battling for their Premier League lives.
Monday's match at the Etihad Stadium begins with United top on 83 points and City second with 80. If City win they would move ahead of United on goal difference while a United victory would leave them six points clear with two matches to play.
"It is probably the most important derby game in my time. We have to go there, make sure we perform in the right way and not give away silly goals," Ferguson said.
"There's no reason why we can't get a good result. There has been an expectancy from City that this could be their decider, but it's our decider too. Either way, it's the decider."
Mancini believes United are still favourites to win the Premier League title because they had a less difficult run-in.
After playing on Monday, United face Swansea City at home and Sunderland away. City visit Newcastle before a home game against Queens Park Rangers.
"They're top, three points ahead," Mancini said. "After the derby they have two easy games. We can talk about the title next Monday night but it's impossible."
Mancini suggested United's experience - the club have won 19 titles - would be a telling factor.
"I don't think they have pressure because they are used to it every year," he added. "For us, it's different because it's the first time. We don't have any pressure because we are not expected to win. But we want to finish well. It's important for our season."
Monday's match is the fourth time the two have met this season. In the Community Shield curtain-raiser United won 3-2 after City led 2-0 at half-time.
City then became the first opponents to score six at Old Trafford since 1930 when they crushed United 6-1 on October 23 before United ended City's reign as FA Cup holders with a 3-2 win at the Etihad in a third-round match in January.
Arsenal lie third on 65 points and are favourites to take the final automatic Champions League place, a position they will strengthen if they can win at mid-table Stoke City on Saturday.
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