Why do people say this? Their second starting 11 is:
Ortega
Akanji Laporte ? Lewis
Philips Rodrigo
Mahrez Palmer? Foden
Alvarez
Our second starting 11:
Butland
Dalot Maguire Lindelof Malacia
McTominay Fred
Sancho Sabitzer Garnacho
Weghorst
There is hardly much difference between them. The keepers are meh, the defenses are quite comparable (one might argue that Akanji is a starter, but same can be said for Dalot), maybe United's is better. The midfield is pretty equal. Mahrez and Foden are better than Sancho and Garnacho. Sabitzer is better than Palmer. Alvarez is much better than Weghorst.
So an advantage to United in defense, and arguably in midfield. City has better attack. Neither squad is a starting XI though, if that was the starting eleven of City, they would be at best, at midtable.