It will be interesting to see what sort of challenge Mourinho plumps for. Because they are completely different types of challenge and pressure, United on the one hand, versus City or Chelsea on the other (to be fair there are also big differences between those two jobs as well.)
At United you have the chance to succeed someone with god-like status, where there is little that can be done to be perceived as transcending your predecessor (sustained domination of Europe would look to be the only real route to this as far as I can see) but plenty of downside risk. In other words, you do well and it is seen as continuity, you do badly it is seen as squandering an inheritance. For that slightly skewed risk profile, he gets to take over the most successful and prestigious team in the land.
On the other hand he could take on a club that will pay him more, give him more money to spend on players, where his success will not always be measured against an almost impossible benchmark, where he will not have to deal with people pining for the past - a past which has been glorious but which will be inflated still further by the passage of time, so that all the bad shit - the refusal to buy midfielders, the 442 dogma, the over-reliance on Scholes and Giggs - will be glossed over or forgotten about, and only the titles, the cups and the persona will be remembered.
Ive said before, if he really wanted to do something monumental, something that would make people sit back and think, wow, now that is a manager who can work miracles, and dispel the notion that he can only do his thing when backed up by unlimited resources, he should take over Liverpool. Awakening that sleeping giant and delivering them a title would be the biggest achievement he could deliver in my opinion, more than fixing a team with a winning mentality at Chelsea, picking up from where Mancini left off at City, or maintaining the success at United. But that isnt going to happen.
On paper, to me the City or Chelsea job would look more appealing to a neutral, someone with no former association with United. But it is hard to quantify the prestige associated with our club, or how he would perceive that. We certainly have more prestige than City or Chelsea, but how that compares to the money depends on the individual. Having just come from Madrid, he doesnt really have much left to prove in terms of managing prestigious football clubs.