Against Blissey, FB Mewtwo has lower DPS than Machamp but higher TDO, as it is not weak to Dazzling Gleam, Zen Headbutt and Psychic. (Entei only has higher TDO if the Blissey has Dazzling Gleam and its DPS is much worse than Mewtwo.)
Relly depends on the relative importance of gym offence/defence (as compared to raids) in your area.