Saturday's clash at Wembley will either see Premier League champions City take a giant step towards achieving a treble of league, FA Cup and Champions League by adding the cup to their championship crown, or United win their 13th FA Cup to complete a domestic cup double, having won the Carabao Cup by beating Newcastle in February.
There is also the issue of local rivalry, with City and United facing each other in a major final for the first time.
Despite their resurgence under manager Erik ten Hag, United go into the game as outsiders to win the cup, despite beating City in their last encounter -- a 2-1 Premier League win at Old Trafford in January -- and having the upper hand in recent cup ties. City are not only favourites to beat United, but also to emulate their bitter rival's 1999 success of doing the treble -- the only English team to pull it off to date.
So can United win, or will City edge even closer to the treble? Here are the key elements in what is the most eagerly-anticipated FA Cup final in living memory.
Is this really the biggest FA Cup final ever?
If you remove club loyalties and take a dispassionate view -- don't expect Liverpool, Arsenal or Chelsea fans to give this FA Cup final top billing -- it's hard to pinpoint any previous one that has carried so much significance and attracted such global interest.
In the 1980s, Liverpool and Everton dominated English football for a lengthy period and the FA Cup final staged two Merseyside derbies, in 1986 and 1989, both of which were won by Liverpool. The 1986 game saw Liverpool win to secure their first league and FA Cup double, with Everton finishing runners-up in both competitions, but there was no treble on the line or the possibility of the other team doing a double.
There have also been heavyweight encounters in recent years between United and Arsenal, as well as Liverpool and Chelsea, but this cup final really does have everything.
United want to end the season with two trophies and win both domestic cups for the first time in the same campaign, but they also desperately want to stop City doing the treble. That's not only because local pride is at stake, it's because the treble is United's unique achievement among English teams and they don't want to share the distinction of doing it with anyone, least of all City.
On the flipside, City are desperate to beat United en route to their own treble. Doing the treble would be satisfying enough, but brushing past United at the penultimate hurdle will only make it sweeter for manager Pep Guardiola and his players.
Man United have prevented a club doing the treble before, right?
Yes. In 1977, United faced Liverpool in the FA Cup final and stopped their traditional rivals achieving the treble 22 years before Ferguson's team managed it.
Liverpool had just been crowned league champions and they would go on to win the club's first European Cup by beating Borussia Monchengladbach in Rome four days after the FA Cup final. But a 2-1 United win at Wembley denied Liverpool the distinction of becoming the first English team to do the treble.
In 1985, United beat league champions Everton in the FA Cup final to deny their opponents a league, FA Cup and European Cup-Winners' Cup treble.
For the first time in history the two Manchester sides will meet in the FA Cup final, but will the blue side or red side come out on top?
Where would it rank among upsets if United beat City?
This is a question that is rooted in the here and now in the sense that City have been top dogs in Manchester for a decade. United haven't finished above their city rivals in the Premier League since last winning the title in 2013 and the trophy count since then -- ironically the year that Sir Alex Ferguson retired is City 13, United 4.
City's recent dominance has turned the rivalry with United on its head -- though United still lead the all-time head-to-head record with 77 wins to City's 58 -- and they are favourites to win this final. Their form is superior: City have lost just once in their last 26 games and that came in the final fixture of the league season at Brentford, with their key players rested and the title already wrapped up.
Historically, United are the bigger and more successful club by some distance, so to suggest it would be a shock if they win on Saturday would seem to be a slight on their tradition and history. But it cannot be disputed that City are strong favourites and a United win would be a huge shock.
Sunderland's 1973 FA Cup final win against a great Leeds United team is regarded as the biggest shock of them all as Sunderland were a second-tier side at the time. But West Ham (also second tier) beating Arsenal in 1980, reigning champions Liverpool losing to Wimbledon in 1988 and relegated Wigan defeating City in 2013 are also memorable upsets.
United beating City wouldn't eclipse any of those, but it would certainly be significant.
Why are City such strong favourites to win?
There are so many threads to City story, and many can be traced back to being owned by Sheikh Mansour bin Zayed al Nahyan of Abu Dhabi.
Since Sheikh Mansour bought City in 2008, the club have been transformed from a perennially-failing team to arguably the best side in the world. City have the best players -- Erling Haaland, Kevin De Bruyne -- and the best manager in Guardiola, but while Sheikh Mansour's financial might makes a huge difference, their period of dominance is all down to Guardiola.
City hired the former Barcelona and Bayern Munich coach in 2016 with the objective of turning the team into the best in England and Europe. With five of the last six Premier League titles, England was dominated a long time ago, while City are just one win (against Inter Milan in this season's Champions League final) from conquering Europe for the first time, too.
Guardiola's intensity and attention to detail are renowned, but it is his ability to constantly improve and re-invent players that marks him out as the best around. Centre-back John Stones has been converted into a midfielder this season, teenage full-back Rico Lewis has been trusted to operate at right-back and defensive midfield, while Jack Grealish, Nathan Ake, Riyad Mahrez and Rodri have all taken their games to a new level.
No team works harder than Manchester City and that is a testament to Guardiola being able to motivate a group of players who have already won virtually everything in the game.
So, how can United beat City?
The good news for United is that they won 2-1 when the teams last met in the Premier League at Old Trafford in January. United have also won five of their last FA Cup meetings with City. But when the heat is on, City have beaten all of their biggest opponents since losing at Old Trafford. Liverpool (4-1), Arsenal (4-1), Bayern Munich (3-0) and Real Madrid (4-0) have all been blown away by Guardiola's team recently, so United have a huge challenge ahead of them.
United simply have to play the same disciplined, organised game they produced when beating City in January. On that occasion, Ten Hag's team cut off the supply line to Haaland by nullifying De Bruyne, but they also had luck on their side with a contentious VAR offside decision giving United a route back into the game after Grealish had scored the opener for City.
Lisandro Martinez's absence due to a broken metatarsal will impact United's defence, but they beat City without the Argentina international in January, so they must do so again.
It's all about stopping Haaland, isn't it?
Not quite. The City forward has scored 52 goals from 51 games in all competitions this season and is clearly the biggest threat to United. The former Borussia Dortmund forward scored a hat trick in the 6-3 win against United last October, so he already has a taste for scoring in derby games. But City are about much more than his goals
A Mahrez hat trick saw them win 3-0 against Sheffield United in the semifinal, and City have four other players who have contributed 10 or more goals in all competitions this season -- Julian Alvarez, Phil Foden, Mahrez and De Bruyne -- while Ilkay Gundogan has scored nine from midfield.
This team can score from any area of the pitch and that is the big problem for United. If they stop Haaland, that's only one element of the job. There are so many others threats to deal with too.
What else do we need to know?
This is City's 12th FA Cup final and they have a mixed record, winning six and losing five of their previous 11. Since 2011, they have won two and lost one, with their last cup final a comprehensive 6-0 win against Watford in 2019.
United will be playing in their 21st FA Cup final, a record shared with Arsenal, but no club have lost more times than the eight defeats suffered by United -- an unenviable distinction shared with Chelsea and Everton.
If City beat United on Saturday without conceding a goal, they will become the first team since Bury in 1903 to go through the whole competition without conceding. Preston North End, in 1889, are the only other team to achieve that.
City beat Chelsea, Arsenal, Bristol City, Burnley and Sheffield United en route to the final; United faced Everton, Reading, West Ham, Fulham and Brighton.