Both of these tragic incidents involved the murder of civilians, in My Lai during the Vietnam War, and in Haditha in 2005 during the Iraq War, by US military personnel.
Though highly illegal, a crime against humanity and in opposition to established military procedures, both incidents were committed by military units in retaliation for recent attacks on their units by insurgents near the massacre sites.
My Lai was committed by a unit of the Americal Division, US Army, while the Haditha killings were committed by US Marines.
My Lai was larger in scale, with between 400 - 500 civilian deaths, while Haditha involved the killing of 24 Iraqis, at least 15 of which were civilians. So I guess the major contrast would be one of scale. There was some question about whether the victims in Haditha had any connection to Sunni insurgents, while in My Lai, villagers were massacred with little evidence of any connection to the Vietcong, or any VC members among them.
Lt. William Calley was the only one convicted in the My Lai Massacre, while a Staff Sargeant Frank Wuterich was still awaiting trial as recently as 2010. Some have suggested that the pattern of military cover up in both cases is essentially the same.