#4 Fanbase
The only geographic locale within which you would find more City fans than United fans is perhaps the confined boundaries of Manchester. There isn't even real competition outside of it; United are the most popular football club in the world, with an enormous (and expanding) fanbase across 24 countries.
To give an idea of the sheer gap in this context, consider this: Deloitte Football Money League 2017 estimates the fan following of Manchester City at 3.9 million twitter followers, 22.6 million facebook likes and 4.2 million Instagram followers.
What about Manchester United? Well, 9.8 million twitter followers, 72.2 million facebook likes, 15.4 million Instagram followers – that is, more than 3 times City's following!