It was certainly a great weekend in the Premier League. I did manage to catch the Blues' destruction of Tottenham in its entirety, but missed United's thrashing of Arsenal. And in some ways, I think it's good that I did because everybody is making way too much of this game. As of right now, United are not even close to City as a team if you take a deeper look at the stats.
The issue is that SOG are more informative than goals at this stage of the season. Because there are more SOG than goals per game and SOG are highly correlated with goals, looking at SOG gives a larger sample and thus tells us more about a team's true talent level at this point in the season. One way we can look at how good a team is at this point in the season then is to look at their goal differential given that the the team and their opponents convert SOG to goals at the same rate, what I call adjusted Expected Goal Differential (the assumed rate is .3 for both teams and the number is expressed per game). Here are the leaders so far this season.
Now obviously, caveats apply due to scheduling. I'm still bullish on Tottenham, for example; they won't play Manchester teams every week (by the same token, I'm very down on Wigan, who got to play the three promoted teams in succession). However, City and United have played comparable schedules (United's was tougher, but they have played two home games to City's one) and City have been far superior. Consider that United have allowed 17 SOG in the past two games and only allowed two goals. That rate of allowing goals will not continue. Through this point of the season, City have been a goal a game better, a significant difference in quality.
Now this is not to say that City will be better over the course of the season. United's true strength is its depth, not its firepower, and I still expect them to win the title. However, people should not be making so much of United's start to the season. It is much less impressive than City's.