I think that it's changed over the last 20 years. In the 70's, make no mistake, Utd had massive support from outside Manchester, even when Utd got 45,000 average. I'd say that in those days, City had the edge when it came to support in Manchester.
Since then, most impressionable kids in the area (and in the rest of the country) have gravitated to Utd, because it's been easy. They tend not to have the close family links and cultural links to the club that City fans have though. Let's face it, if you are 25-40 years old and you're a City fan, you deserve a medal for sticking with the club during your formative years. You probably did it because you either had no choice (your old man insisted on it), or your particular group of friends, also influenced by their fathers, supported City. Consequently, our support was essentially a 'local' thing and more to the point, you probably actually went to the game.
The 'typical' casual football fan in Manchester under the age of 40 is probably more likely to 'support' Utd. That support amount's to watching it on tv. The 'typical' City fan, whilst probably fewer in number are more likely to actually go to the game and actually turn up for a parade, because guess what, they actually give a fuck.
Outside the City (and this is a guess), I'd suggest that those who are City fans largely have a family, social, educational or historical link with Manchester, whilst those who are Utd fans, in huge numbers are glory hunting horrible shithouses with absolutely no link with the area and visit once a year on the annual 'pilgramage' with the East Anglia supporters club et al.