On the face of it, the evidence clearly suggests Manchester City have a more efficient defence than United. City conceded 33 goals last season, which constituted the Premier League's joint-best defensive record, equal with Chelsea, and four fewer than United. Joe Hart won the Golden Glove with 17 clean sheets and kept 29 in all competitions, setting a new club record. Carlos Tevez scored 20 league goals but it was a defender, Vincent Kompany, who was named City's player of the year.

The paradox is that if you put it to a vote to find the best six defenders from the two clubs then, individually, the majority would almost certainly come from Old Trafford. Nemanja Vidic and Rio Ferdinand would be automatic choices. Patrice Evra has pushed Ashley Cole hard for the right to be known as the best left-back in the league. Chris Smalling and Phil Jones are both ahead of Micah Richards when it comes to England's right-back spot, despite the improvements in the City player since Roberto Mancini became manager.

Kompany would be the one City player who automatically warrants inclusion – but possibly the only one. Mancini is still not fully convinced by Joleon Lescott and thought about replacing him in the summer. Richards's failure to win the trust of Fabio Capello can aggrieve City supporters, but has never really inspired outrage on a wider scale. Kolo Touré is trying to re-establish himself after his six-month ban. Gaël Clichy has looked an astute signing but carries a reputation for lapses of concentration. Aleksandar Kolarov can be vulnerable to pace and endured an erratic first season.

So how come City have the superior defence? Mancini's team have restricted their opponents to 59 shots in their first eight league games, the best figures in the division. The crazy thing is that United, top until last weekend, have the joint worst, admittedly having already faced Chelsea, Liverpool and Arsenal. Ferguson's side have faced almost twice as many, 108, and the same as second-bottom Wigan Athletic. United have had fewer shots of their own, 103. City, with 134, have managed almost five to every two.

For a mid-table club to be conceding the most shots would be unusual but for the champions to be so generous is freakish and, though there are mitigating circumstances such as the injuries to Ferdinand and Vidic, it reveals much about the different structure of the two teams.

The most striking distinction is that Ferguson does not have one classic holding midfielder, whereas City's defence has the insurance of Mancini playing two in every match. Both managers prefer the same 4-2-3-1 system but the two for United – for example, Anderson and Michael Carrick – are very different types of player from Nigel de Jong and Gareth Barry at City.

Mancini devotes hours to this part of his team, sometimes not even using a ball in training routines aimed at ensuring his players work as a unit and that each one knows precisely the space he should occupy. "He has come from a country where defence is No1," De Jong says. "That's what he preaches. That is always his message: 'Make sure we don't concede, realise that a clean sheet is holy.' If we can do that, he knows we have enough quality to score goals."

Roberto Martínez, the Wigan manager, noted after his side's 3-0 defeat at the Etihad Stadium in September that City actually operate with "a back six" and that, even when they are dominating games, they seldom stray from a policy of safety first. "Apart from Clichy, that six have been playing together for a while now, and Hart looks as though he has been playing for them for 10 years," Martínez said. "When they have the ball, they always have five or six bodies behind it. Even if the movement is flexible they always look very solid. They play to a style and when they have to defend, they defend."

The best defensive performances of Ferguson's tenure can probably be traced back to Carlos Queiroz's time as assistant manager. Many of the players found Queiroz's methods bogged down in specifics – "It was as if he didn't want us having too much fun in the week so we'd be hungrier on Saturday," Gary Neville writes in his autobiography – but it did produce results. Queiroz's daily routine before the Champions League semi-final against Barcelona in 2008 was to put sit-up mats on the training pitch to mark exactly where he wanted the defenders to be to the nearest yard. "We'd never seen such attention to detail," Neville recalls. "We rehearsed time and again, walking through the tactics slowly with the ball in our hands." United went through over two legs, with Barcelona not scoring in either.

But Queiroz left that year and what United have now is a more cavalier approach. Ferguson noted recently how when Basel turned a 2-0 deficit into a 3-2 lead in their Champions League tie at Old Trafford last month the Swiss side scored their second goal while Evra was on the wing, Jones had wandered into central midfield and Ferdinand was the only defender inside the penalty area.

For Mancini, that would have been a sin. The Italian's ethos can be gauged by the way he could not be fully satisfied after the 4-1 thrashing of Aston Villa on Saturday, or even the 5-1 against Tottenham Hotspur at White Hart Lane in August, because his side conceded a soft goal on each occasion. Ferguson, in contrast, tends to laugh about his team's capacity for making life difficult. "Manchester United cannot defend," he once said. "We just can't play defensively. The fans, the people here, won't stand for it. It's just not United."

Not strictly true, of course. Ferguson would not be the most successful manager in the business if he did not fully understand the importance of great defending. Vidic, for example, is a formidable opponent, a centre-half who gives the impression he would run through a plate-glass window for his team.

This is also the club where Edwin van der Sar went from November to March without conceding a league goal three seasons ago. Yet the current side have been unusually lax so far this season. Mancini's training-ground mantra is based around the same thinking that once prompted George Graham to say: "The biggest sin in football is to make the goalkeeper do any work." Ferguson's is different. United, he says, have "a kamikaze streak, which in a funny way I quite like".

Read part one of the derby series, on goalkeepers