I would say sacrificing a cop for a mafia would be a good trade, as there are more citizens than mafia, but you guys play how you want.

The only way the cops in this game are playing it well is if they haven't found any mafia yet. That's all good as it is partly chance, and there is that stupid godfather to worry about. Once a cop finds a mafia then that mafia should be killed within no more than two days. I honestly don't remember who the cops are so I don't know if they found any mafia yet.