I would say Coin is worse - yes snow is terrible, but he was really just supporting the games because it was such as tradition at his time. But then Coin who'd lived through the terror and knew full well what they were like from the other side and yet she still suggested it.
I also agree that Coin is worse because Snow admits he is evil, he lives and relishes in his evilness, but Coin dsiguises her evilness. She tries to act good, when in reality she's just as bad as Snow. (I loved it when Katniss killed her instead)
I'll throw out a topic. Who's better : Coin or Snow?