I am not underestimating it, I am just trying to put myself in Mourinho's shoes. Also, it might not be the question of City Vs United as I don't think Fergie's job at United is finished yet. The team we have at our disposal is a work in progress and I would be disappointed if he leaves it midway. It would be better for United if Fergie leaves when the likes of DeGea, Smalling, Jones, Rafael, Cleverley, Hernandez, Welbeck etc. have matured. I don't see any other manager achieving the same results that he does with the present team. Giggs, Scholes, Rio, Vidic, Nani are all nearing an end of their United careers and the rest haven't won much. Sir Alex needs to see some of the boys in this team become men before he calls it a day. The job at United, imo, should not be available for another couple of years and I don't see Jose sticking out with Madrid for that period.
That could be said of any United team. The truth is Fergie isn't going to manage forever and its quite rare for a manager who had previously committed to retiring by 60 to manage well into his 70s, particularly when he's well aware of what happened to Jock Stein. Whether or not Mourinho stays at Madrid would probably not matter given that he could easily take a year or two off before taking another managing job.