It's the story that will define this transfer window and one that promises to make history: Where does Paul Pogba's future lie?

Is it worth it for Manchester United or Real Madrid to break the world transfer record to get him? Or should Juventus stay strong and not give into the pressure to sell their star?

ESPN FC looks at the pros and cons of signing Pogba for the three clubs involved.

MANCHESTER UNITED

Pros

A mood change: Regardless of the cost involved, the world record signing of Pogba, particularly in addition to the appointment of Jose Mourinho and free transfer of Zlatan Ibrahimovic, would generate huge enthusiasm after a series of distinctly un-United years of disappointment. On paper at least, they would look like they're back again just when they need to be, with Pep Guardiola, Antonio Conte, Jurgen Klopp and Mauricio Pochettino upping standards of the Premier League.

The Happy One: Mourinho would have liked United to have wrapped things up earlier, but Pogba's arrival would mean he has got the four players he targeted this summer -- Pogba, Zlatan, Eric Bailly and Henrikh Mkhitaryan.

The missing piece: Neglected toward the end of the Sir Alex Ferguson era and not upgraded adequately enough under David Moyes or Louis van Gaal, United have stumbled from Marouane Fellaini to Bastian Schweinsteiger in the center of midfield, but they would be there at last with Pogba. The total midfielder, Pogba would be the missing piece to United's midfield.

Pogboom: Along with Ibrahimovic, Pogba is one of the most marketable players in the world; a one-man brand. Signing him is expensive, but he is also a money spinner.

Cons

Doubts over Woodward: Whenever United's executive vice-president Ed Woodward ventures into the transfer market, there is a sense he is getting played by the selling club and/or the agent. An inordinate fee would only further that reputation.

The expense: Perhaps Pogba is overpriced; there was better value to be had this summer. United's dithering meant they missed out on 18-year-old midfield dynamo Renato Sanches, for instance, who moved to Bayern Munich for £27.5m from Benfica.

Power shift? As if super-agent Jorge Mendes' influence at United wasn't already great enough, Mino Raiola would now have three of his clients (Ibrahimovic, Mkhitaryan and Pogba) at the club.

REAL MADRID

Pros

The Duodecima: Winning a 12th European Cup surely wouldn't be beyond a team with a midfield of Pogba, Toni Kroos and Luka Modric. Nor would reclaiming the La Liga title for the first time in five years.

Hit back at Barcelona: Andre Gomes who? After Barca hijacked Real's move for the Euro 2016 winner, the impact of signing Pogba would more than make amends -- it would eclipse anything their rivals have done. Real fans are overdue a galactico signing after a quiet window by their standards last summer.

Relationship with Zinedine Zidane: Zidane became the seventh man to win the Champions League as a player and manager and is a big admirer of Pogba's. While Mourinho pushes all the right buttons to get players to reach another level, you wonder if the Juventus midfielder might respond better to Zidane, under whom, one presumes, he would get more freedom of expression.

Cons

Midfield not key: "I'm happy with the squad I've got. It's difficult to improve," Zidane said recently. Reinforcing the midfield isn't a big priority, not with James Rodriguez, Isco, Casemiro and Mateo Kovacic around (for now) as well.

No balance: As we saw with France at Euro 2016, Pogba is wasted in front of the defence. His best position, left of centre, is where Toni Kroos operates, and Zidane would be reluctant to move the German.

The lesson of James: Buying a player before thinking about where he fits in is a Real trademark, but maybe the £63m the club spent on the underperforming James is enough to make them think twice.

JUVENTUS

Pros

Team building to win UCL: Massimiliano Allegri has told Pogba to think very carefully before leaving the Old Lady, because he believes Juventus are now one of the top four teams in Europe. Two years after reaching the Champions League final, they now think they have the ability to win it, and keeping Pogba would give them an even better chance of bringing the trophy back to Turin for the first time since 1996.

Sending a message: Juventus are not a selling club. Bayern, Barcelona and Real don't accept offers for their best players; why should Juventus? Retaining Pogba and bringing in the money spent on Gonzalo Higuain through the sales of fringe players like Roberto Pereyra, Simone Zaza, Hernanes and Stefano Sturaro, though difficult, would send a strong message to the continent that Juventus are not a breeding ground for bigger clubs.

The complete midfielder: Not everyone was sure about Pogba getting the No. 10 shirt at Juventus. He is not a classic No. 10, but by the end of last season he showed himself worthy of it; 11 of his 12 assists on the season came in 2016. A complete midfielder, why would you let him go if he is willing to stay? A midfield of Pogba, Miralem Pjanic and one of Sami Khedira or Claudio Marchisio [when fit again] is one of the best around.

Cons

Missing the deal of the century: Sometimes the head has to overrule the heart; sometimes an offer is just too good to turn down. All the stars have aligned to make this the deal of the century for Juventus, and a return of more than £100m on an initial investment of £800k represents a great piece of business. No one thinking rationally would begrudge Juventus for taking it.

James covers the Italian Serie A and European football for ESPN FC Follow him on Twitter @JamesHorncastle.