Maradona believes the pressure of chasing a long-awaited top-flight title is starting to take its toll, with Mancini having forced his side to start retreating into their shell.
He said: "My heart says Manchester City, but I believe Mancini, with the players he has, is not daring enough.
"He's a bit too defensive. Maybe because he is Italian.
"As an organisation, and with their style of play, United are more aggressive. But if I have to support one team, I'll be a fan for my son-in-law."
Maradona has also claimed that he would rather have seen Aguero stay at Atletico Madrid last summer, but appreciates that it would have been difficult to shun City's advances.
"Seeing Aguero in Madrid would make for much more interesting matches than at Manchester City," added the current boss of Dubai-based outfit Al Wasl.
"But he has finally found a place in the world and a team where he can fight for important things."